Johnson Plastics, JDS Industries, Gravograph.
Johnson and JDS will send catalogs. Johnson carries a wider variety of products, JDS is very good too, gravograph is a place you order from if you need their product, which happens from time to time, but I don't know many people that use them as their primary supplier.
Make sure what you order is designated for the laser. Those materials are two ply and have a very very thin top cap sheet. If you get the non laser versions their cap sheet is much thicker, so your laser won't deal with it in one pass. You can engrave it, you'll just have to use multiple passes, so stick with the laser friendly stuff.
IPI plastics and Rowmark and the two big ones out there. Both sell swatch books. I'd suggest getting them.