It depends on the brakes whether they're unobtainium or not. Shimano EF-500 is available in 3x7 and 3x8, but they're long pull so you are limited to V-brakes (on your frame). The old EF-51 and EF-65 used to come in a "four finger" version that was compatible with cantilever brakes by moving a spacer, but those are indeed impossible to find. I've seen references to the new EF-500 coming in a four-finger version that accommodates cantilevers, but I haven't seen those for sale anywhere either. But if you're going to go with V-brakes, those shift/brake combos are available.