Is there a shop where I can get replacement foam cushions for a sofa? The cover material is fine.

Why not try a hardware store ? They do sell some type of foams to isolate pipes and that turned out to work fine. I'm not in your area but you surely must have a homepro or Bonthavorn. Good luck.

There are several place along Sukumvit from Tai to Nua, on both sides, that sell materials for upholstery. Some of them do car upholstery. 

 

I think along Thrappasit also.