At the risk of sounding like a smart-alec, have you considered learning how to do it yourself?
I know this kind of thing isn't for everybody, but it really isn't that hard, and for a small investment you will save yourself a ton of cash down the road.
I buy my tires on-line and change them myself. If you ride a lot, it is a skill worth learning. There are lots of how-to vids available on youtube.