I can't point you to any stores, but you would probably want to look for a sign shop that makes signs out of plastics. They may be willing to sell you sheets of sintra for market price.
The difference? Sintra is hard. It's plastic. I don't care what chemical it is or what process is used to make it, the important thing about it is that it's hard. Solid. It's a sheet of plastic. Foam is soft, squishy, rubbery, bendable. You can roll it in a tube and fold it with ease. Regardless of the similarities in the manufacturing process and chemicals involved, I consider them very different animals because they behave differently, have different starting and ending properties.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com