Duncan,
Have you tried Coda? www.codamount.com. Had a supplier in St. Louis that had a very nice hard foam core, closer to Gator in strength but still paper.
If you do your own framing, if you have a tax license, and if you do bulk, you should identify and contact a local framing supplies distributor. United Manufacturers (Google) is one national wholesale distributor and they will ship cases of what you want. They will sell to you wholesale.
Items like foam core are expensive to ship due to large sheets and 'dimensions' so it is better to have a local distributor pay for them to be trucked to them and then delivered to you.
My local distributor delivers for free to me or I can pick up at their warehouse.
www.hrws.com -- don't know yet how the prices stack up to the other links here, but they're great folks. Their web site has about 1/10th of their inventory, order the catalog on first order.
Steady Hand is right -- the shipping will kill you, particularly on small lots. Foam core and matting have oversize charges and just aren't worth mail ordering generally except in case lots. HRWS will mix matts to make a case.
