www.bpstrings.com www.juststrings.com
These two sites offer the widest selection for shortscale strings, especially flatwounds.
For the old school sound, I'd go with the shortscale GHS Precision Flatwounds. D'Addario makes a couple of sets in their Half Round and Chromes lines that are cool. There are even tapewound flats made by Rotosound and LaBella. You can get just about any string type or gauge in shortscale if you look in the right places.