I don't know if it helps but WeLoveMacs has the glass bezel for the 24" however it does not list it as a part for the 20" you could call them I suppose. Here is the webpage http://www.welovemacs.com/9228180.html($109.00) which is not bad plus I am guessing the 20" would be cheaper. But it sounds like the store is your best bet. GOOD LUCK
FYI: when I talked to apple about a repair on my iMac and they said I could have home service I told him that I would rather just take it to apple since I've heard (from a apple genius) that it is really easy to get dust behind the iMac glass---he then said that they use something similar to a lint roller before they put the glass back on so that no dust is left behind...i thought that was interesting but decided to take it to the apple store anyway.