I gave up trying to find Saltpeter Ore in Infinity modpack. So I used the alternate method - I actually needed Thermal Expansion Niter (so I can make Gelid Cryotheum), but I believe Railcraft Saltpeter is an equivalent to Niter. (that may or may not be correct, I don't know for sure)
The alternate method is to TE Pulverize cobble into sand, then combine 2 sand into 1 sandstone using a TE Cyclic Assembler, then TE Pulverize the sandstone into sand with a secondary byproduct of Niter. Use the TE augments for each machine, and set them up in a row to process your cobble automatically - within hours, I had over 6000 Niter using this method and the augments that provide a higher percentage of secondary products.
Aha, sorry, I see your edit. Oh well, I tried. In Infinity, on an SMP server, I had a heck of a time just finding a Desert biome, never mind finding Saltpeter in a Desert. I collected all my Saltpeter Ore from the Nether, where it seems much more plentiful because it isn't restricted to a single biome in the Nether. But I know that doesn't help you either.
Good luck.
Which modpack are you playing, by the way? If it is Infinty I will load up my game and fly around the desert to see if I can find any Saltpeter Ore - at least you'd know if CoFH messed up the oregen or not, if I can find some.