You might want to call around to the different vendors. I know that Pantera Parts Connection & Wilkinson's have them, for starters.
The prices for new ones are nosebleed high. However, given the number of people who have changed over to 17" or 18" wheels, there have to be a lot of discarded 15" wheels lying in garages all over the continent. I'd be surprised if someone didn't want to sell their old wheels to help pay for the new ones.
If you wind up with used wheels, they should be stripped, dye tested, stress relieved and re-sealed before you run them.
Let me know how your search turns out, because I'm looking for a couple of reasonably priced wheels to use as spares myself.