Your best bet is to probably scour the internet (craigslist, ebay, etc) for a cheap wheel. A trainer can go through tires pretty quickly if you use it a lot, and so I don't know that you would save much money by not getting a wheel in the long run. Plus, when you factor in the idea that you would be compromising on the type of tire you would like to run, I just don't know that it is really worth it. Less wear on the wheel (though there would hardly be any anyway), tires, and cassette if you had a trainer-specific wheel. For me, when I got new wheels, it was a no-brainer to get a cassette that was in the ball park of a good tire in terms of cost so I could have a trainer wheel. Look for someones stock takeoff that they just replaced.