Holsters are so individual I will just mention that I currently have a Aliengear and IMI holster and have been happy with both.
As an aside don’t be afraid to try a PPQ 9mm 4” holster either. I have found that they all (so far) fit the P99 just fine - in one case better then it does the PPQ apparently! Mine are the AS version not the QA but I don’t think there are external differences?
I would suggest you take a look at Vedder Holsters, Dara Holsters and Stealthgear USA holsters. I have used all of those and still do. There is also a holster thread within the P99 section with some great options as well.
My personal favorite IWB holster (and I own a few) is my Black Arch for my PPS but last time i checked they did not cater to the P99 but I hear the PPQ 4in works (been meaning to try it). Oddly, the best holster I own for the P99 is from willowhawk27on eBay.
As it's nearly impossible for a civilian in Germany to get a carry license for a real gun, I mostly have my blank/pepper cartridge P22 with me, in cold weather sometimes my blank/pepper P99. Fits in my Fobus paddle just like the real one, and as I have to conceal it, I don't need any more extra safety than the simple (but adjustable) retention.
I was also surprised that the Chinese knockoff for the P99 also works fine and didn't break so far, as I paid only approx. 7$ on ebay for both gun and double mag holster.
Of course this only works with coat, wide jacket or XXXL shirt, as they are OWB.
I would second the Vedder holsters. Leather on the skin side and that plastic stuff on the outside. I have one for the PPQ 45 APC and love it. Another for my summer carry gun a PPQ [email protected] 9mm. Both are fairly large guns, but I can wear them all day long. I got the combat IWB for both.