I am a rather avid collector of cam models who pee and found the best site for it by far is Cam4 where you will find women and sometimes couples engaging this in open chat while others try to stick to privates. You can search for tags like #pee, #pipi and so on which will narrow it down quite a bit for you. Other sites where I have seen it, stripchat/xhamsterlive which use the same system just different sites and chaturbate but you will most likely have to go private to see it.