If it would help, here is the link to a thread, and the text to my message (message #278, if I recall) about the exact same problem with my 05 Bambi. I found one at a local RV center. I called Jackson Center and they sent a replacement in a few days, but I had to get a new one quick because of the weather. I'll guess I'll use the one JC sent when it disintegrates again next year. I treated the new one with some UV inhibitor, so it may last a few weeks longer than the original.
Vent Cap - FYI
I came home yesterday to find the rear vent cap on my 2005 Bambi broken away from the bracket and leaning against the air conditioner cover. We had some mild winds earlier in the day, but the vent was closed. Upon closer examination, the reason it broke was obvious. The plastic lid was brittle to the point it crumbled between my fingers. The trailer is still under warranty but, based on my past experiences, I had absolutely no expectations that I could make arrangements to get it fixed before the next weather event. I immediately e-mailed Airstream to find out if the part was an industry standard, or if I would have to get it from an AS dealership. As expected, it's over 24 hours later, and no response.
In the meantime, I photographed the vent cap with a digital camera, and took it to the local RV supply store. They had a replacement part that looked just like mine for under $17, and said if it didn't fit, I could return it. It turned out to be the exact replacement part. It was a bear to get on because of the close proximity to the AC unit, but it took about 30 minutes with some help.
For those who may experience the same problem, the part is made by Elixir Industries, and is called a "Jensen New Style Lid" - # 17-J7291.
NOTE: It was the cheapest vent cap in the store.