Another vote for the Valhalla 2500 series. I have mine hooked up to a Fluke DC calibrator and thus I have a current calibrator (0.01%). My 2500 will do 2A and will also do AC current (with a supplied AC voltage) up to 10 kHz.
The build quality, unfortunately, seems pretty average. I've attached a picture. A few op-amps, a few precision resistors, and that's about it.
I see a few currently listed on eBay, but the prices are absurd (several hundred dollars asked). I wouldn't pay more than about $75 given what is inside.