To my knowledge, the 2 backstraps that come with it are the only available ones. There are Ameriglow night sites out there, and some of the forum sponsors carry them for a pretty good deal (True Hunter Supply).
Holsters: check out the holster sub-forum further down on the main page. If you also elaborated with what you were looking for (OWB, IWB, Tactical drop leg) would help. I have a Crossbreed Supertuck for my FNP-9 and like it a lot.