As others have said try a truck parts outlet, they're called Rain Caps, and you can get them in Galv or Stainless. You can also get ones that are silent and don't make a clanking sound when closing. You will need to provide the OD of the pipe, off the top of my head the smallest I think there is, is around 63mm.
You may also have to cut off that bit at the end. Check whats available first.

