Paired 20", 20 spoke wheels are available from www.velocitywheels.com look in the drop menu, under 'recumbent' wheels. The set you'll want will probably be the Uriels. In order to be sure you're ordering the right size, look to your current tire set as a guide. 20 x 1 1/8 (or other English fractional sizes) denote 451mm wheel size, whereas metric 20 x 1.5 style demarcations are 406mm wheels. Dahon Rolf and the-name-of-the-newer-set escapes me wheels will not fit most bicycles as they're axle flange size is for their propietary fork with a 74mm over the locknut dimension, in place of the more standard 100mm.

That said, Dahon Rolf wheels can be found at www.bikepartsusa.com and www.ahoycaptain.com (look under Dahon parts).