You may want to drop a line to ikenfixit.com or maybe look on ebay. Since it's just one key, don't get finicky if you need letter b and find only the a.
Im looking for a single key for my Compaq Presario V2205us. I need the ctrl key for the left side of the keyboard. Neither Compaq or HP will sell me a single key so if anyone can help I would really appreciate it.