Here is a list (non definitive) of all the places I buy my materials from. I get asked for the info a lot so hopefully this will help some of my readers.
Perspex, Poplar Plywood, Birch Plywood, Polypropylene and MDF
Kitronik, a great supplier who label each sheet clearly, check out their electronic things too.
Real woods > 1mm thick and 1.5mm thick MDF
SLEC, they even refund some of your shipping fees if it cost less to post.
Always Hobbies, now has 150mm wide materials and a good range of flavours.
OakDale Crafts, a great value ebay store.
Engraving Laminates, Laserable Rubber
Hobarts, they also have a wide range of perspex but my last batch of Birch ply was not so good.