I used the frame of an old Fuji 10 speed that I got for $80. It's pretty light, and I got it in great condition from a former bike mechanic. Try your local craigslist. Look for bikes or frames people are selling and feel free to put up a wanted ad. That's how I got my frame. Check out garage sales too for old 10 speeds. Just find one that isn't heavy. It might take a while to find the right frame, but don't lose hope. It took me a couple months and four different frames before finally getting the one I have now (I only payed for one of the other frames though). One thing to watch out for is the type of cranks. Try and get cotterless cranks. The second frame I got (and first one I ever built up all the way) had cottered cranks and they started giving out within 30 minutes of riding. It was a mid-70s Peugeot, and apart from the cottered cranks it was nice. It was pretty light, and I know they have other Peugeots that have better cranks, so you might look for one of those.
If you don't want to go through the possibly long process of finding the perfect old 10 speed, you can get a full bike off of bikesdirect.com. You can get an entry level track bike for about 300-350. They're definitely not the best bikes around, but they aren't that bad from what my friends who have gotten bikes from them say. Toe-overlap with the front wheel is definitely an issue with them though (as with any track bike).
