Building an SS is a hell of a lot easier to build than a proper (reliable) FG so I'd say convert your own, just about any fairly modern and complete bike you can get your hands on will work and meet your requirements. It's nice to have the ability to adjust chain tension via dropouts but it's not necessary with a SS... a modified derailleur will work nicely to take up chain slack, it just doesn't look as 'clean' and lean.