Has any one replased bearings on an Eames swivell dowel base  

Perhaps one of these
will fit your chair:
http://www.swivel-chair-parts.com/flatSwivels.html

If you
dont use the original Seng bearings, the chair won't swivel correctly.

Why not?
What's special about the Seng swivel?

Seng produced the swivel for HM
Seng out of Chicago produced the swivel for HM

Yes, I know.
The question to Td was "Why do you say that the chair won't swivel correctly if a non-Seng swivel is fitted to it?"

If
You can replace the entire swivel mechanism with another one that fits (maybe one in the link that fastfwd posted) the chair, there is no reason why it wouldn't work.

the question
is about bearings, not replacement swivels.

Okay then
Why couldn't one find bearings of the exact same size/type?

Ah, I see. Wrong terminology.
I understand what you're saying now.
However... If the bearing [that's the name for the whole swiveling assembly] is so worn or abused that the balls [those are the things that roll around inside the bearing] have fallen out, why would you want to put new balls back in? They'll just fall out again, right?
And are the Seng bearing races designed to be separated, to allow the balls to be replaced, then put back together? If so, that'd make them the only cheap thrust bearings I've ever heard of whose components weren't meant to be permanently pressed together, and it'd mean that the Seng company must have made bearing-rebuild kits available for these cheap barstool swivels... Which seems moronic.
Still, you never know. I guess I'll have to track down one of those chairs and take a look. Maybe it'll surprise me.