Not sure what your definition of cheap is...I found this in about a minute with a google search.
BlackBerry Storm2 9520/9550 OEM battery cover - Type 2 - Cellular Nationwide Network - Cellular
I am sure there are additional out there. $18 seems cheap enough for a $180 handset. And you will be MUCH happier with your new S2. It is a world of difference.
Good luck