Hi nodpete -- Hope you're enjoying the Forums! You're going to make us look good with this question.
That item is called j-trim and some Airstream dealers stock it. For quick service and inexpensive postage I can recommend OASIS R.V. CENTER, INC. - Home
in Tucson. They know Airstreams top to bottom and their parts department is excellent
working with you over the phone. 1-800-658-5863
You'll need to remove and replace pop rivets to put in the new material but it's worth it -- it looks sharp! You'll have to snip the longer hidden backside with side cutters every inch or two to go around the curve by the windows.