We did a similar project a few years ago and had a good experience a few years ago with The Famous Foam Factory in Berkeley. We also had them make a removable cover for our foam mattress with fabric we supplied. Three years later both the mattress and the cover are holding up well. They carry a large selection of different foams, and will help you select the type and thickness appropriate for your project. They're located at 2397 San Pablo Ave in Berkeley and you can call them at 510.841.9001.
We've written about Foam Order before; they have locations in San Francisco and San Rafael, as well as online ordering.
We've also heard good things about Bob's Foam Factory in Fremont, but don't have any experience with them ourselves.
Anyone else have a recommendation for Adriana?