I tried talking to a local art framing store to see if they could do automated foamcore cutting. They couldn't, but he was willing to sell me foamcore at $4.50 per 32x40 sheet in packs of 5, which was a bit cheaper than my local office supply store. I didn't ask if it was black or white foamcore. I haven't taken him up on the offer yet either, as my current foamcore pile hasn't run dry.

The moral of the story is that it might be an avenue to try. You can also ask around to see if they have off-cut pieces 6x6 or larger that you could have cheap/free.

I've been making a couple dozen tiles by cutting 3" tiles out of the center of 6" tiles, saving a little on foamcore. It doesn't help a ton, but . I haven't noticed any issues with warping yet, though the tiles have only been around for a couple months.

This reminded me of something that came up in a search a couple of years ago and got me looking for it again. Dollar Tree is a discount store in the USA that sells a lot of the "Made in China" stuff for $1 USD.
They have some 20" x 30" black foam board in cases of 50 pieces for $50 USD with in store pickup.


I have no idea of the quality of the foam board sold there, but could be something to explore. Maybe if there is a Dollar Tree near enough you could check out the foam board there and see if it would be a good investment for you.