There are some very good suppliers around... many of which have well-qualified service departments, including:
ACE... highly reputable. I've used them (in one form or another) for various services and supplies for over 20 years.
http://www.cinequip.com/
Other very good companies that I've done business with are Hadden Theatre Supply (Louis Bornwasser),
http://www.haddentheatresupply.com/
and Claco Equipment and Service (Mark Gulbrandsen)...
http://www.clacoequipment.com/
I know there are other good ones out there, but these are the ones I've had direct experience with.
There are also a lot of theatre owners, managers and projectionists who frequent the various forums around here, and have a lot of experience in the technical side of this business. It's not hard to get a lot of opinions... and a few really good answers!