You buy field corn. You should be able to find it sold at local hardware stores, grain feed stores, actually in any store that sells birdseed. You don't want to use the corn that we eat, like you said, it would be wayyy too expensive.

Cheaper yet is the feed corn we feed the deer with. It comes in 100lb bags and is about $7.00 That would fee alot of squirrles. My squirrles love it as do the Doves,Chipmunks, Blue Jays, Turkeys, etc. Tractor Supply sells it or Farm and Fleet. Of course you may need to live in a podunk town to have one of those. LOL
Lisa

I buy Squirrel corn & all bird seed/supplies from my local feed store as well as food for my pet birds.
It's always fresh plus they deliver for free so can't beat it!