I was able to find a local plastic supply warehouse (through lots of googling) and found they have scraps and things, but it is unpredictable and they don’t do online biz. In my experiences a lot of places like that are pretty old school. Also, the local Dick Block had a fire sale on balsa foam, not sure why. I find going to the local shops you can usually get the deals. Same with foam core. Took me forever to find a place that would sell a few sheets of 4x8 foam core and not charge an arm and a leg, but finally found a local sign shop that was wiling.
Hoping someone has some better answers though!