You could make one. It doesn't see boost so you just need to prevent it from allowing unmetered air in.
Auto stores sell sheets of gasket paper. Or you could lay on a thin layer of RTV around the outside edge with your finger and let that dry so it won't extrude inward when you tighten the elbow. Don't tighten the nuts much, and use blue locktite to keep them from backing off with use.
I could never find this gasket. I was calling it a plenum gasket but every time I asked for that the parts stores would pull the gasket that goes between the supercharger and lower intake. I ended up just making one with rtv.