Rob:
Try these folks,
Stoney Creek Equipment .
They advertise in PB magazine and carry both windmill and electrical compressor aeration systems. You should be able to get a rough idea of prices.
If you talk/correspond to the pros here at the forum (Ted Lea/Cary Martin/Sue Cruz/ etc.?) they should be able to size a system to your pond. I have gathered one wants to ensure one doesn't have too big or too small a system for ones needs.