I also encountered an unusual fitting in my bathroom remodel. You can try looking up the fitting on a manufacturer website like Nibco - www.nibco.com, they make plastic fittings.
I found that to actually buy the fitting, I could not go to HD or Lowes, I had to go to a couple of pipe supply stores until I found one that did enough volume to stock the part I needed, a 4" elbow with low heel vent inlet.
In my area, Ferguson was that store - I think they are a nationwide chain of plumbing supply houses. If you take the fitting into the store, they may be able to help you. This is definitely one area where the service at a plumbing supply house is worth it. Plus, they may have doughnuts if you go early enough.