Probably fewer these days than in the past, since the breeding industry has expanded with sport specific bred horses, and demand the high prices for them, and the trainers get the incentives to send their rich clients to buy these horses in the commissions paid for the sale.
However, just because someone pays more for a prospect does not guarantee success. And the dark horse, purchased cheap, is still a diamond in the rough. Through a number of decades, Ive known a number of horses who have come from a murky source, off Indian reserves, off the range, off the track, out of someones back yard. They are still out there, but fewer astute trainers beat the bush to find them any more. Instead, they get a client to buy something expensive, and get paid the commission for that, AND the training/riding fees. So make more money doing it that way. And this method backs up the claims of the breeders of sport specific breeders, that you HAVE to have X breed of horse to compete at the top levels of sport, and you HAVE to pay big bucks to get it. In truth, you don’t need either of these things, you just have to be a horseman, and purchase the right horse to get the job done, whatever type of horse that may be.