Could this "bottom bracket" be a derailleur hangar? (do a google search for pics.) The part that holds the derailleur (rear shifter) to the bike's frame? If so, they are proprietary, and you are pretty much at their mercy.
If the bike was taken in for a shifting problem, there's a good chance the hangar is bent or cracked. Happens when new bikes are damaged in shipping, or when accidentally dropped on the right hand side.