Perhaps answering my own question, I ordered one of these (it was $84, including delivery, during Black Friday) in order to properly characterize ultra low power energy harvesting circuits: https://www.aliexpress.com/item/33031557247.html?spm=a2g0o.cart.0.0.6f673c009dbs4n&mp=1 Ignoring its multimeter aspects, what remains is essentially a precision CV CC micropower supply intended for use as a calibrator. It has both microamp and microvolt output resolution, and the advertised accuracy (0.015% to 0.02%), if true, seems pretty good for the price. I should be able to calibrate a uCurrent Gold with it, for example. It may not arrive until much later in the month though. If anyone else wants to get one too, the normal price through this vendor is around $104, which to me still seems like a good price. I'm not sure why it's currently priced at $153, unless maybe it's in temporary short supply following the big Black Friday sale. PEAKMETER PM7221 Professional Digital Calibrator DC Voltage Current Digital Calibrator 0-10V 0-24mA Voltage Tester $153.00