Since Adobe brought out version 2.6 of AIR, the links for AIR 2.5 got removed. Luckily there’s an archive for old versions of AIR:

http://kb2.adobe.com/cps/853/cpsid_85304.html

Here the direct download links:

http://airdownload.adobe.com/air/lin/download/2.5/AdobeAIRSDK.tbz2

http://airdownload.adobe.com/air/mac/download/2.5/AdobeAIRSDK.tbz2

http://airdownload.adobe.com/air/win/download/2.5/AdobeAIRSDK.zip