Depending on what yer tryin' to do, there are several bearings available at yer local parts store that will do it.
There are also companies such as Bearings and Drive, or Dixie Bearing, that can fix you up. You just need the dimensions of the bearing you need, ID, OD, and thickness. All measured in inches. As far as I know, there are no metric bearings. (at least I haven't seen any)