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.
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.