LBS. In this town (I don't know if an endorsement of Eugene bike shops would really be helpful) it seems that every shop has some kind of a track hub. I just went for the cheapie. And any freewheel hub can be made into a quick ghetto play-around version; a standard threaded cog (of the sort that fits onto a BMX or track hub) will thread on fine, & then a bottom bracket lockring will sort of hold it on. Then it's just a matter of adjusting spacers on either side of the hub until the chainline is straight, then re-dishing the wheel to something close to centered.
Be aware that the cog in question is, through some fluke not entirely clear to me, only going to fit a BMX chain; the same chain will fit your old chainring, mostly just meaning that you need a new chain.
http://www.63xc.com/ -- has some good articles, particularly under 'howto.' Although I consider Sheldon to be the definitive.