I got some 4 mm OD, 2 mm ID brass tubing from Amazon. I'll see if I can find the link for it.

The 4 mm OD is slightly too big to fit into the collar for the Chaparral-type feedhorn that Tim is selling. You would have to remove a small amount of the OD. I did so by putting a 2 mm OD stainless steel rod through the brass tubing and turning a little off the brass tubing on a lathe. The brass by itself is too thin to turn.

73, Zack W9SZ