Originally Posted by base2

Any of the Soma frames with the Tange IRD sliding dropouts could easily be run as a disc braked single speed. It really couldn't be easier.

Soma Fab

Rush, Juice, Jawbone. Buena Vista. Maybe some others... I have a Buena Vista disc. But, it comes in a rim brake version, too.

Finding a canti or V-brake single speed may be a bit of a tough go. The reason is that the horizontal dropouts on well thought out single-speeds tend to be sort of close to perpendicular to the brake bridge to minimize pad adjustments when making chain adjustments. Canti/V brakes don't tend to lend themselves to big changes in wheel position.

Good luck with your search.