Here is a link to a number of responses to that popular issue -
For contractors, depending on where you live and how common sprinkler systems are in your area, you may be looking at a fulltime sprinkler system installer, a plumber, a landscaper who installs systems, a landscaper or even greenhouse/plant center that installs and maintains them, or an irrigation contractor. You may have to yellow-pages for the proper names in your area, then use Angie's List to see what their reviews are.
In estimating cost, bear in mind annual spring rehab and startup costs (including a couple of replacement heads, at least, at $25-50 each installed), and possible repair of sections that had water left in them and froze and split. If you do not blow the sytem out with a large (towed size) air compressor in the fall and you have ground freezing conditions, that will happen. Budget blowing the system out each fall to avoid that - about $100-150 typically to winterize the system.