Baldor, or Glentek come to mind.
There are a couple of considerations to bear in mind when trying to make a fast moving machine. You did not say what the load would be like, but usually people want pretty short acceleration/deceleration ramps so the thing will be up to rapid speed quickly. This requires a fairly large amp to supply the juice to do this, otherwise you get nuisance tripping of the amp on overload. Servo motors are pretty tough, and can stand brief applications of high current. Best go bigger on the amps and smaller on the motors, rather than the other way around, IMO. Of course, having plenty of $$ makes this easier to decide
Also, don't go "ape" on the encoder resolution. Depending on your controller, it can only handle so many encoder feedback pulses per second. So an ultra high res encoder can actually cap your rapid speeds at a lower rate, if your controller cannot handle what it is putting out.
A lot of controllers can read 4000 pulses out of a 1000 line encoder, so do your research on the controller before you buy.
I know that a lot of Galil motion cards can handle 12,000,000 pulses per second, but some of what these other guys are using for controllers, can only handle maybe 4,000,000. Whatever the number, divide this by your encoder pulses per rev, and this will give you your max motor rpm. How long a duration that the acc/dec ramps are is totally dependent on matching your motors and amps to the load.