The showerhead actually begins upstream from the joint you display, with the first nut that screws onto the pipe emerging from the wall. It is probably
a standard 1/2" Female Pipe Thread nut, which you will also find on just about any replacement showerhead. If for some strange reason you actually want
to replace only the parts you have removed, you will probably have to find an identical showerhead and dismantle it to get the items you need.