A battery grip is a must have for photographers who shoot frequently in portrait mode, and nothing beats the quality of genuine Canon products.  Up for sale in this eBay auction is a Genuine Canon BG-E5 Battery Grip in Perfect Condition for Canon Rebel XS, XSi and T1i, still under warranty from B&H.

Take a look at the auction now:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220473331169