On my old cv, a 1999 GMC, I purchased a kit from this place. http://www.thecruisecontrolstore.com/ez-catalog/X380183/0001
I installed it myself, everything was in the kit. They also have on their web site some authorized installers. The one I was going to use was in Michigan, north of Detroit. But I did it myself in a few hours. You install a second stick on your steering column just below your turn signal switch. It worked as well as the factory cruise In my current cv.
If you're through the St Louis area give Auto Air Co on S. Kingshighway a call, they've been in business since I can remember, at least 50 years or so installing aftermarket air cons cruise controls and such. Quoted me $399 I think to add one to my 1999 GMC Van. Thinking of upgrading to a Sprinter later this year, so going to give the CC a pass.