Sealed bearings are pretty standard. Skateboarders replace them all the time, as does inline-skaters. That's one of the secrets to speed in those fields is to replace the cheap bearings that come on skate wheels with higher quality precision bearings. Some bike shops should also stock some of these bearings as well. You specify the axle-diameter, the OD and width and should be able to find one easily. Specialty shops that carry specialized tools and machining equipment, nuts & bolts should have them.

Otherwise, there's numerous online mail-order places tool. Like McMasterCarr, start on page 1015.