The old shower arm has developed a crack and it's weeping water and
getting worse. I have to replace it before it damages the wall.

I got it unscrewed easily, but what I found wasn't what I was
expecting. From all the descriptions I found on the net and from
looking at the replacement arms available in hardware stores, I
expected the arm to screw into a fitting behind the wall, sticking out
through a hole which can be covered by a flange. Instead, I found this:

http://home.comcast.net/~esionder/temp/showerarm1.jpg

A pipe sticking out into the shower. Attached to that is this:

http://home.comcast.net/~esionder/temp/showerarm2.jpg

http://home.comcast.net/~esionder/temp/showerarm3.jpg

Maybe I have my terminology wrong - which is the "arm", the copper pipe
or the chrome cowl? Anyway, I would like to find a replacement for the
chrome piece to save myself from hacking open the wall. However, I
can't find any similar piece in any physical or on-line store. Can
anyone suggest a source for this piece? Any other suggestions on other
ways to repair this?