Here is a list of my holster selection for my FNX-40s&w and FNS-9mm. Now most nylon holsters like Uncle Mikes "side kick" shoulder holster will fit both guns.
These are some diverse holsters and all fit the guns perfectly.
The Galco paddle holster for the Sig P226 fits the FNS-9 perfectly and the FNX-40 "cocked and locked". I stretched the top leather strap to fit the FNX to fit with the hammer down. Very easy to do.
The Blackhawk SERPA for the Springfield XD is a bit loose for the FNX-40 but two small pieces of moleskin will make the holster fit perfectly and quiet. You can find the camo moleskin at most archery shops.