I am assuming that you are talking about a crankshaft main bearing. If so, this is bad news. The bearing is not available seperately. The reason being, my guess, is that the case halfs are cast, the bearings inserted, the parting surfaces machined, cases bolted together and then the mains are finished align reamed to ensure that they are straight. This would eliminate any variances in the case to case dimensions. That is why they are sold as a pair.
Now, if I remember, the front camshaft bearing for a GM 6.2 diesel is very close to the id. of the crank bearing, do not know about the od. It would be a machine shop operation to remove the bearing, sleeve or bore the case to accept the GM bearing and ensure that it is installed dead straight because there would be no bearing material to remove if reaming was required. It sounds pretty expensive, to me.
So, the bottom line for your choices are:
1. Repair with ??? results.
2. Purchase new cases.
3. Find another engine.
I have to ask....what happened? Run out of oil? What other damage was done?