On somet holstermakers' websites (i know safariland does this for sure), they have a compatability chart or an "also fits" section. This is mostly useful for injection molded and certain leather holsters.
From my searches for a holster for my FNP, I remember that selection is geared more towards sig, glock, beretta, and various 1911's.... But I think I remember some safariland Sig and some other companies' H&K USP holsters fitting the fnp. research the sites and figure out what fits... Or email their customer service departments
To answer the obvious question, I settled on a BDS Tactical universal leg rig (nylon, not poly) for other reasons.
Good luck! Enjoy your fnp!