Most my horses, I just either stumbled into ownership of them (had them given to me for one reason or another) or heard about them through word of mouth from friends.
Denny, Jesse, Bessie, Rafe, Taz, Nester, Diamond, Talyn were all free horses that were offered to me (Denny in lieu of training bill, the rest straight out of the blue).
Dobe, Koda, Jet, those were all horses that friends of friends had for "sale" to a good home that could do something with them. Of those 3, Koda was the only one that had been handled but he wasn't even halter broke. The most expensive was Jet, but he was a registered (and very well bred) APHA...and he still only cost $500 before the horse market crashed.
If you're just looking for a prospect, those can be found nearly anywhere. Listen to your local trading radio station, browse your local craigslist and horse-for-sale sites, check out the flyers at the local feed store and tack shop. Look around for any larger breeding farms that might have production sales once or twice a year.