fpnibs.com
Besides Bock, Jowo (and all the branded variants) and Schmidt it's not uncommon to use Pelikan, Esterbrook or even press fit vintage nibs/feeds. The biggest hurdle is mating up with ink delivery because most won't have a nipple to mate up with a cartridge/converter. If you go away from the big 3 German nibs, be prepared to get creative with anything from eye dropper filled to ink sacs, threaded converters, etc... You'll have to look to some of the vintage pen ink reservoir designs.