Well, unless the owner is in desperate financial difficulty and just wants the horse off the feed bill, it's unlikely they're willing to drastically drop the price or give away the animal.
I got my lovely young TB for free, but he needed to be let down and retrained. Which cost me a boatload of money.
So, you're either going to pay up front for something already trained and ready to go, or you'll pay for it in the long run when the animal needs to be trained for the discipline in which you ride.
Either way, there's really no such thing as a 'free' horse.