Unless I misunderstood Streets post (entirely possible!) you still have to have the tires on hand to try. I had a buddy who owns a tire store loan me some new ones but there's other ways to do it.

Most any tire store will let you pull some out of their "disposal bin". You know, the ones that are bad, bald, or blown. You're not putting air in 'em anyway so overall diameter and width is all you're interested in.

You could probably "rent" or borrow some from a tire store. Maybe explain what you're doing and agree to buy your tires from 'em when you're done.