Occasionally you see wheelsets on Ebay but I think you're better off getting some wheels built. Velocity make a lot of their rims in 650c. Another option is that when I needed rims I often found them for dirt cheap at places like Nashbar etc.

Worth looking around.

You can often find some very dope TT/aero front wheels for cheap (in 650c sizes) second hand and anywhere with Triathalon stuff should have rims, tires and tubes for you.