Personally I'd buy a fixed-specific wheel (actually have one made) but you can get an old wheel and thread on a fixed cog (suicide conversion coz you can't fit a lockring), redish the wheel (recentre the wheel as you don't need to allow for a 5spd+ cluster any more)
http://www.sheldonbrown.com/fixed-conversion.html#hub
Personally I'd buy a fixed-specific wheel (actually have one made) but you can get an old wheel and thread on a fixed cog (suicide conversion coz you can't fit a lockring), redish the wheel (recentre the wheel as you don't need to allow for a 5spd+ cluster any more)