Consulting online reviews is a great way to find a reputable air conditioner dealer. Angieslist, Yelp and other review sites can offer great insight on what level of service you can expect. Experienced businesses that have been established in an area for a significant period of time are usually reputable.
Going to a factory authorized dealer and repair shop for the specific type of air conditioner you want can be a good step towards making sure you are using a reputable dealer. If someone close to you has recently had a system installed, then ask him who he used and if he was satisfied with the experience.
If you are having a hard time finding a dealer in your area, then you can order air conditioning units online. Facebook profiles of local dealers can offer some reviews for your appraisal. The Better Business Bureau may have reports on some air conditioner dealers. A good Better Business Bureau rating is a sign of a reputable dealer. Large home improvement stores such as Lowes and The Home Depot are good resources for air conditioners. Always look for a dealer that offers a warranty for both labor and materials, and does not mind answering your questions.