I've had good luck with Airstream Supply
http://airstreamsupply.com/aluminum-drip-cap-1-3
Vintage Trailer Supply has them also but they're painted.
Make sure the dripcap i s a few inches wider than the window itself, especially on the escape window. Would also recommend you consider mounting a little higher than AS did, at least on the older models. You can't slide the window out of the hinge with the drip cap in the stock location. At least you couldn't on mine. Ohio built.