Santa Monica Plastics – Nice people with a good selection of plastics, used to be Hastings till they built the subway to the sea through their old shop
Bohnhoff Lumber – For solid wood and MDF, only quality stuff here, south of Downtown
All Electronics – They have so much great stuff, everything I have needed electronics-wise they always have seven different types. Also once I thought I had blown out my chiller because of bad advice I got at Frys and they totally took the time to help me, ended up being a 50¢ fuse replacement in the plug, saved my bacon! In Burbank
Foam Mart – For all your EVA foam / cosplaying needs. In Burbank
SCI-Arc Supply Store – Used to be an independent store but has now moved back into the SCI-Arc building so actually visiting might be a little harder but they have good stuff, especially for laser cutting, small pieces of acrylic and relatively flat sheets of corrugated cardboard as well as PETG. Downtown
Sawmill Creek Forum – As forums go this has pretty good advice on cutting technique and materials, just make sure to cross-check with other sites, some people really don’t know what they are talking about 😛