You could go to a machine shop and buy some .001 or thicker shim stock. Wrap a piece of it around the bearing and add red loctite. Then re-install the bearing in the hub.
Someone d to advertise a fix for this in the back of dirt bike mags. It involved boring out the hub and shrinking in some sort of collar.
I would think that if this is done properly it should be as good as new. I have no idea where to look for this, but hopefully this gives you some idea to start your search.
Everytime you lube your chain you should "wiggle" your wheel to check for bearing play. But I guess you know that now!
I bought a new rear wheel assy for my 01 125sx when I lost the bearing froze up during a race. It included hub, wheel all laced up and new bearings & spacers for my for $200. from the KTM dealer and it was an excell. I think it was a "take off" from a new bike that they had tricked out with new rims and hub.
If you can't get it repaired, the cheapest place for a new replacement i have found is Service Honda. $340 gets the complete rim with bearings and seals installed.