There are more and more places online to buy a cheapish rear track wheel. There are a number of wheelsets around for about $120. Some are cheaper than that, but I would shy away from them. Good cheap hubs for track are made by Dimension and Formula.
But, be aware that these wheels come spaced and dished on center, and so when you are converted a road bike you will likely have to either tinker with the bb/crank (to bring it farther in) or else respace and redish the rear wheel off-center a bit. The first alternative can sometimes be easy and cheap, but can be fairly expensive. The second alternative undermines much of the motivation for buying track wheels.
If you are just getting into it, I believe it makes a lot of sense to simply convert the wheel on the bike (if it is a freewheel hub, and not a cassette). The so-called "suicide hub" option is not the best, but it is done by a lot of people without trouble. And it is free if you can convert it yourself or fairly inexpensive if you have to pay someone.
jim
Last edited by jgedwa; 10-17-07 at 09:53 AM.