Most craft places like Hobby Lobby will have some type of foam blocks or pieces but they'll be expensive for what you get.
Check with a place called Tractor Supply and ask about foam blocks they get from trailers? I've read about that on here in the past, and they sometimes give them away for free (so I've seen; they usually trash them). Should be close to the size you need if not larger.
Also check boat/marine supply places; I believe they supply REALLY large (white bead foam) pieces they use for dock floats. But no idea what you'd pay there.
Thanks. Never knew of tractor supply and the trailers! I know the dock places do carry some large pieces that are not the crumbly kind. 4' x4' x 18" deep are $80 so not terrible. I call tractor supply in the morning!
From what I read, they get the trailers shipped to locations, and they'll be shrink wrapped and then stablized with big foam blocks which they just need to trash or recycle so good chance they'll be happy to get rid of them... found the mention!!