Anything that works with the ppq m1 will work with the p99. You could also custom order from vedder.
Vega makes a very comfy iwb holster that offers no retention but a very low profile. Most holsters designed for the ppq m2 will also work.
I usually carry this one Fierce Defender IWB Kydex Holster... https://www.amazon.com/dp/B07DCWST8L?ref=ppx_pop_mob_ap_share
I carry either the p99 or the ppq sc.
Pretty much any holster made for the ppq m1 will work, and they are much easier to find.
Fierce Defender IWB Kydex Holster... https://www.amazon.com/dp/B07DCWST8L?ref=ppx_pop_mob_ap_share try this one out. I’ve found the retention to be good