There are lots of guys who are certified and have mobile operations. They are the ones who come out and service tractors and equipment at farms. I know a couple of guys who do that.
Or you could try to hook up with someone who does maintenance and repair for a local school district. Offer him or her some side money on the weekends.
Of course if you have to have an actual brick and mortar shop, I'm sure the yellowpages are full of them. Or just head towards the interstate and look for a truck service center.