[bobH]

Around my area, it's as simple as a trip to the local tire store. For me, I just tell them, 'four rollers', off the used junk pile. Sure, they're radials, and sometimes miss-matched sizes. If I'm not in a hurry, I can ask for four matched sizes. Last time, it was four pretty good tires off a Camry, to use on an old Ford. Time before that, I decided to buy new tires for a Grand Cherokee that I have, and the take-off tires were put on a 38 Plymouth. Sure, a little too big. But, they are great tires, and are still on that Plymouth today.
I'm currently in the same situation as you... a project that I bought, came with four deteriorated flats. I'll be getting four of almost anything, off the used junk pile. Good enough for rollers, and good enough for the first 'neighborhood trial trips' that I hope to do within a year or so. Tires are free, and I pay for mounting (No balance).