If you're sure it's an internal bearing bottom bracket, then you probably have something like an Alivio FC-M431

I'm betting it will be cheaper to buy a whole new crank than to try to track down the chainrings for this, if they're even available.

Also, I think there's a newer version of that particular crankset, the FC-T4010 IIRC. Still Octalink, still 9-speed 48-36-26.