A professional trainer or horse-person will be able to access your skills and determine if you and the horse are compatible. Prof. Roadapple is very saddened by the fact that horse sellers often sell an untrained horse to an inexperienced rider just to get their money. These unscrupulous people don't care that the new owner and the horse may suffer. That's where an experienced friend can really help keep you out of trouble. IF you have a truly experienced helper, another source of potential horses is at an auction.
Auctions are where most people end up buying terribly unsuitable horses.
It's very easy to buy the wrong horse because it was cheap and pretty. This is a very a common practice, and beware of the seller who refuses. This checkup will let you know of any problems the horse may have and also may develop at a later date. This will give you a good understanding of the horse's health and help you determine if the animal is worth the asking price. That answer depends on breeding, training, and popularity of the horse or it's bloodlines.