Vortex keycaps show up from time to time on mechanicalkeyboards.com, but Leopold doesn't seem to sell theirs separately anymore. They used to, though.
Tai Hao also makes doubleshot PBT keycaps, and has been for far longer than either Leopold or Vortex, but they seem to get a bad rap around here. They're fairly thin, on occasion their space bars are a little warped, and they insisted on using a horrible antibiotic coating on their keycaps for way too long, but if you like their color combinations, they're good for the price. I do wish they'd make a simple white on black set. They make a white on black doubleshot ABS set that's dirt cheap, but not a PBT version.