Locally (Canadian West Coast) we can get bulk magnetic sheet from our local plastics shop - they use it and sell it for signs and such.
It's bee a few years since I bought any, but it came off a big roll, sold by the foot.
Check your local yellow pages for "Plastics Suppliers" or something similar, ask about "magnetic vinyl". Our local shop (called, very creatively, Industrial Plastics) is a treasure trove of bulk styrene sheet, foamed PVC (Sintra), magnetic sheet, random bits of plastic tubing and and all sorts of nifty stuff.