If possible, have the dress altered where you bought it. It will not be the least expensive option, but it assures you a certain level of quality while giving you some protection in the event that a problem emerges in the alternations. Once you take the gown outside the salon, the store is not going to assume responsibility for anything that goes wrong. The best salons have a staff working on the gowns within the building. Some charge a flat fee for alternations, while others price them a'la carte.
If you find the dress in a city other than where you live, making two or three extra visits for your fittings may not be practical. In that case, check the designer's website to find out where he sells in your area, and call that salon to see if they'll alter the dress.
You can also go with an independent alternations specialist. To find an experienced one, ask at your city's best fabric store - they'll often keep a list of local tailors. If there's a local wedding gown designer, ask if he does alternations on other designers' gown - some do and are very skilled at it.