Where can I find a 30x47x15 tapered...

...roller bearing? (upper steering head bearing for fork swap).

got a precision bearing up around those parts?

How about a 30x48x15 and enlarging the hole by .040" (removing .020" from the wall)?

If that would make the wall to thin, you could weld a ring around the outside to strengthen it.

The simple answer is to have a machine shop make a new stem and press it in. I know you said you don't want to do that, but it shouldn't cost much and then it would bolt right in to the standard bearings.

An industrial bearing supplier such a CBS (Canadian Bearing Supplies) should carry one. We also have a Lou's Bearing in London. I've always purchased my bearings from CBS. Tapered roller for steering heads are an arm and a leg from Kawi. Pennies from CBS (not literally but relatively so).