There are BX, B, L and M series Kubotas going from smallest to largest chassis size. First thing, gotta get the exact model number... is it a BX2350 (i.e. a small "subcompact" size machine)?
If that's what you have, you're looking for a non-OEM backhoe... probably Woods makes the most common ones, model BH6000, for those small tractors.
Finding a used one is a pretty good trick... not many people buy one, modify the tractor for installation and then change their minds and decide to sell just the hoe. Search Craig's and Fleabay, check some of the online classifieds like MachineryFinder, Ironplanet and Fastline, maybe call around to ag dealers who handle the Woods brand.
Good luck!