I asked Jerry Dunlap the same question. I would reccommend getting to know the guys at your LHS, and asking them if they can order something. If you get in with a boat club (like me) you can all pitch in and get something good, too. Here's what Jerry Dunlap wrote to me.....
Jeff
I use 65% Wild Cat fuel. It has 18% lubrication and I usually add a couple of oz. of castor oil from Sig.
I'm not advocating 65% fuel, but I do know the .21 size motors like the nitro. Wild Cat also makes 45% fuel. Mike Z. likes Byron's fuel and it comes in various levels of nitro. There's lots of good model fuels, Red Max, Power Master, O'Donnell's.
Jerry D.
Oh by the way, your K&B will give you plenty of trouble free use, no need to switch to a different powerhead. I would run on the rich side, then lean gradually. I would rather have my boat bog a bit on the top end of the revs, than stall out completely and waste most of my day. It's better if you let it bog on the rich side for the sake of the piston and cylinder, too. They lean out during the course of your run.