You might find that if you're into shitty porn, the standard tubes are not going to have the type of content that you'll enjoy. Some of them even ban the uploading and sharing of poop-related videos, so many have decided that the best way to counter these issues is by having a site completely devoted to scat porn videos.
One such example is Scatrina, and it's a standard tube style location that hosts hundreds of scenes showing off real pooping action. The site has a mixture of straight-up shitting as well as some scat play in erotic contexts, so whether you enjoy the voyeur aspect of asses ejecting shit or prefer the dirty practice of fucking with it, this place is for you.
Getting around is easy and if you sign up, you can share videos yourself. The biggest uploader by far is a guy called Scatman2 - I suggest you check out his scenes, because this guy obviously knows a thing or two about scat porn.