When the first info on the Quads went out, all distributors were informed to stock up on fins. So try contacting your local distributor first.
If that doesn't work, for the fcs fins I think you can pretty much take whatever as long as they are symmetric and look reasonable similar to the original ones. The minituttle fins are more complicated. There are some that make custom, like Tarifa fin company. And Witchcraft thrusters are minituttle. But these are very different from the original fins. Stiffer and with a much bigger tip. So I would only get those if nothing else works out (or if you actually like to take the chance and experiment a bit.
If worse comes to worse, I have two personal spare sets (and the fcs fins too). I'm not to keen on letting them go (since this means I have to order new from Thailand which means quite a big freight cost) but I was thinking of ordering some 13cm front fins to try so it's not that big a deal. The one I have is the 16+11 combo, but that works on the 76 too and actually can be a nice addition (meaning you have the possibility to run 15+11, 15+12, 16+11 and 16+12). You can reach me at firstname.lastname@example.org