In case anybody else is wondering, I found a very nice impeller inside of this vacuum:
http://www.homedepot.com/h_d1/N-5yc1...atalogId=10053
It is only a $20 vacuum, so I have no qualms about tearing it apart just for the impeller. The impeller is the kind normally found in larger shop vacs. It is made of sheet steel, and is 5" in diameter and about .3" thick. It is enclosed on both sides. The intake hole is a bit under 2" diameter, and it has a 5/16" hole for mounting to a shaft. The impeller easily comes off by clamping the motor still with vice-grips and unscrewing the nut that holds it on.
We haven't tried attaching it to a kit motor yet (need to turn an adapter), but so far, I highly recommend this impeller.