Go to a couple of trailer sellers (horse or RV) and get recommendations on shops that install hitches. Then get some written estimates on what you want.
But before you do anything find out just what kind of truck you have and then what your capacity will be. Don't spend money on a 10,000 pound rated hitch if your truck is rated for only 5,000 pounds. The Trailer Life website has listings in the Towing Guide for trucks back into the '90s. For older trucks there's a e-mail address you can write to for more info.
As previously mentioned, know you vehicles tow and load rating and go by that.
Most bumper hitches are in different ratings/sizes up to 10,000#. You can also find hitch extenders and higher ratings, but you'll be paying $600+ for those. If you go higher you also need to look into higher load range tires and shocks.
A "normal" Class III hitch is usually what is needed to pull a 2-3 horse trailer. Check out trailer weights too - they will fool you. A Class III will set you back about $300-$400 with electric. BE SURE to look into a brake box too to control the trailer brakes- another $100 but well worth it, and I know mine come with lifetime warranties- they'll replace them free.