I have a friend who approached this in a way that worked pretty well. He went to a nearby college and talked to one of the fine arts instructor. With the advice of the instructor, he commissioned one of the students to create an outdoor sculpture. It was a win-win-win situation. My friend got a reasonably nice commissioned piece on the cheap. The prof used it as a way of teaching the student. The student got some cash for his semester project.
The only drawback I saw was that one has to make sure that everybody's vision is in sync. I have also seen this kind of thing go bad, where the student wanted to create something a bit more out there then the customer wanted, and it ended up in the garage -- everybody was pissed. The customer ended up with a $3000 doorstop and the student was upset because her work was hidden and not appreciated.
Generally, though, if you are in sync with the instructor, neither vision nor quality are much of a problem, as long as you not looking for a museum piece.
You might also be surprised if you ask around to find out that you have acquaintances or friends who do large scuptures. Over the past couple of years, I have run into three acquaintances who do pretty good work out of their garages. One is a mathematician who does geometric sculpture, one is a forensic graphics person/crime scene reconstructionist who does commissioned representative art, and a third who is a mechanic who does these strange welded nuts/bolts/gears thingies.
billo