Home Depot, aka The Man Store, carries a variety of interior doors that will work fine. I have gone through 3 of their Luan Mahogany doors (hollow core, interior doors). They run less than $30 and are straight and smooth. I usually put ceiling tiles (celotex) on top of the door so I can pin my sticks to it easily. I know some of the guys use drywall, but my old fingers aren't what they used to be, and celotex is much easier on the ancient digits.

Good luck and good building.