I haven't been checking the listings so much as of late, but I think you'd do well to find a donor watch for under $1000. The days of $500 V72s are gone forever, I'm afraid.
You have a few ways of approaching the search. You could use search terms "valjoux 72" and expect to pay top dollar for the listings you find. Alternatively, you could search through the hundreds of items that come up when you search under "vintage chronograph". Look through all the triple register chronographs, and learn how to tell a Valjoux 72 from 71, 7736, and other movements with a similar layout. It can be tough, especially if there are no caseback-off pictures. Finally, start writing up a list of watch brands that made chronographs with V72s. It's still possible that someone will list a "Clebar Chronograph" without mentioning that it's powered by a V72, though it doesn't happen too often nowadays. Sellers have brains, too.
Finally, steer clear of movements with any visible rust, or that may be in questionable condition. If you find a non-running V72 that's missing a few parts, run -don't walk- away. The price of repair (if it's even possible) could easily double your costs. I've seen incomplete movements sell for more than $500, so either the bidders knew exactly what they were doing, or they were hopelessly optimistic. Either way, you don't want to be party to such auctions!