It's not particularly cheap, but someone's selling a Sturmey Archer 3speed fixed hub on STW
http://singletrackworld.com/forum/topic/hub-gears-for-your-fixie-1
EDIT - not actually fixed, my bad. Still a good way of putting hubgears on a 120mm spaced frame
@umop3pisdn started
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It's not particularly cheap, but someone's selling a Sturmey Archer 3speed fixed hub on STW
http://singletrackworld.com/forum/topic/hub-gears-for-your-fixie-1
EDIT - not actually fixed, my bad. Still a good way of putting hubgears on a 120mm spaced frame