I am looking for any Chess AI for flash CS5 ActionScript 3.
A quick Google search reveals much of what you're looking for, including good discussions on the basic algorithms involved. If all you're looking for is a solution you can plug into your Chess game, don't expect anything terribly robust, especially if you're looking to outsmart DeepFritz or Rybka.
Should you be interested in the process of programming a Chess AI, you might want to read through the Chess Programming Wiki.
(As Drknezz points out, http://www.gnu.org/software/chess/ is also an excellent point of research for anyone interested in chess engines.)