The following places to buy materials are suggestions bordering on recommendations. They are curated from past good experiences; no site or company paid or traded for their listing. You are free to buy your materials elsewhere as long as they comply with the allowed materials list.
If you need it “today,” you may wish to try RFP Plastics in Kennewick. They have a limited selection on hand, in a variety of colors and thicknesses, but are able to order in some things they don’t have. However you may be able to get a decent amount for a great price if you’re not picky. They can cut things down to size for you. They’re friendly, helpful, and honest.
RFP Plastics
908 W Canal Dr
Kennewick, WA 99336
Tu-F: 8am-5pm
Sat.: 8am-12pm
If you don’t mind waiting or want a larger selection of colors at your fingertips, check out ePlastics (Ridout Plastics) down in California. They can also cut things to size for you, but charge a fee for this service. However if you order multiples of the same sheet size then you get a discount. Do not buy anything outside their Plexiglas section, except for Delrin, as you will not be allowed to laser it here.
Some suggested categories to look in are listed below. Make sure to double check the thicknesses you add to your cart.
If you’re ordering a lot of acrylic you might find it cheaper to use their labor charges page to pay for your cuts all at once instead of individually when you add a sheet needing cuts to your cart. Be sure to describe what you want precisely. If there is more than one way to make those cuts you might not get what you envisioned. Also make sure to select send remnants as any leftovers can be useful for testing or little odds and ends.
If you're interested in paint-free colored engravings in acrylic, then Rowmark products are a great way to go. It offers a variety of color pairs suitable for signs, badges, and more. Delvie’s Plastics out of Utah is a nice place to order this brand of acrylic and they offer more than just Rowmark. However for brevity only the Rowmark pages are linked below.
Confluent advises staying away from the front-engravable satins, mattes, ultra-mattes, and value mattes as they do not engrave as nicely as the LaserMax and seem to curl at the edges when cut due to melting. This means engraved areas may not go through the top color as uniformly and will appear even shinier than with LaserMax. At least in the case of the standard (not ultra) matte, the non-"metal" LaserMax has a very similar reflectiveness and is an all-around better replacement.
Blick sells this kind of linoluem in a variety of sizes and mountings. You can safely laser smaller pieces out of larger ones. If you happen to be in Seattle, there’s a Blick on Broadway. (Other art stores in Seattle are also very likey to carry this product.)
If you wish to buy it elsewhere, just make sure it is the same material, made with linseed oil and so on. Avoid things like Speedy Carve, Speedy-Cut, or other materials that do not tell you what is in their product. Also do not buy the modern vinyl floor coverings sometimes wrongly called linoleum, as they are generally made with PVC and dangerous to laser.
Leather is relatively easy to obtain, but Tandy Leather offers a wide variety of sources, sizes, and finishes of leather. They seem to frequently have deals and have been known to offer a full hide for over 50% off. However be aware some leather processes may put things like chromium or other products into the leather which may make them undesireable.
The best place to get plywood locally is Windsor Plywood in Kennewick. You can buy 5′x5′ sheets of Baltic Birch there as well as 4′x8′ sheets of whiteboard, and more. They even sometimes have “hobby” blocks of various woods useful for other woodworking projects. If you can’t cut down something that big yourself, be prepared to wait and to pay for it. It can take them a few hours to couple days to get to your order so don't expect a same-day pickup. This is especially true if you order on a Saturday.
Windsor Plywood
311 N Van Buren St
Kennewick, WA 99336
Tu-F: 7am-5:30pm
Sat.: 8am-4pm
For hardwoods, you can get various domestic boards from Rockler. They may not be the cheapest, but the boards are good.
The aptly named VeneerSupplies.com stocks quality veneer in a variety of domestic and “exotic” varieties. Almost everything they sell is by the lot and each lot has its own photo. Really handy!
If you want cheap glass cups to try engraving, go check out any local Dollar Tree. These will typically be soda-lime glass and produce a fuzzy look when engraved. Target has a different variety of glassware but it’s more expensive, of course.
Looking for borosilicate glass? You can get it from McMaster-Carr in rectangular plates. Fair warning: it's expensive. They also sell other shapes and types of borosilicate glass or quartz, but they're even more expensive. Borosilicate glass will engrave very differently. It seems more suited to lines and hatch-fills than completely engraved areas like soda-lime glass.
Borosilicate bakeware is rather hard to obtain inside the US aside from labware such as beakers, flasks, and so on. This is because the US rights to the name Pyrex were sold off and the new owner uses soda-lime glass instead. Therefore, all glassware with a lowercase “pyrex” label is soda-lime glass but glassware with an uppercase “PYREX” is borosilicate glass. Buy from Europe or try trolling garage and estate sales if you're looking for one of those elusive PYREX borosilicate baking pans.
Inventables sells anodized aluminum sheets in a variety of colors from 3″x3″ to 12″x12″, though not all colors come in all sizes. We have a metal cutter capable of cutting metal up to 2mm thick (~.08″) to any size you desire.
You might be able to find other thicknesses and sizes for cheaper on ebay, but quality is unknown and Inventables seems to have the most interesting color selection.
Johnson Plastics Plus also sells anodized aluminum sheets, tags (luggage, pet, etc.), and a variety of other trinkety gift items.
Pretty straight-forward suggestion: OnlineMetals.com
They were at the Seattle Mini Maker Faire in 2017 (at least) and seem pretty down with the whole “maker” thing. In addition they also allow you to buy any size of rectangular plate by entering in your own dimensions. The laser has been used to mark up some kind of brass obtained from them using CerMark and it worked out pretty well.
Do note CO2 lasers cannot easily mark metals so a compound such as CerMark is required. It is sprayed on to the metal, left to dry, lasered, then washed off. Anything not lasered will come off during washing. Not all metals are suitable for CerMark. If you want your own, you can get it from CerMark Sales.
Stainless steel and brass should work just fine with their main LMM6000 formula. If you're going to be marking aluminum and tin, buy the LMM14.
Crystal River Gems sells great agate slices and also ships very quickly after ordering. Start on this page of their website. Make sure to add a cherry-pick for every slice you want to suggest some aspect of. It is recommended to cherry-pick to avoid geode holes or request rounder slices, if possible and wanted. Be aware your cherry-pick requests can overide your color selection if you specify something like “natural” as that will result in you getting brown.