Buy Through a Horse Dealer
Many dealers are excellent, but sadly there is the odd one who is not so scrupulous. One advantage of buying from a reputable dealer is that they will often allow you to have a horse on a week's trial.
There are also dealers who specialize in different types of horse, such as showjumpers or young event horses. There are also innumerable horse sales and fairs held Throughout the country, which range from the 'don't touch them at any cost' to the immensely respectable and reputable.
It is not advisable to buy a horse from a sale, unless you are extremely knowledgeable or have an expert with you. It is not worth being tempted by the thought of getting a real bargain; if a horse is being sold at a poor-quality sale, there is usually a very good reason.
On the other hand, there are an increasing number of performance horse sales taking place. These sales are strictly regulated, usually offer a warranty, and are a good place to see a lot of quality horses going through the ring in one sitting.
However, the first-time buyer should treat sales with caution, and always take someone experienced and knowledgeable with them.