Can you be more specific what you need? What ranges, what kind of accuracy / stability / noise / linearity, control interface, speed, new/used, budget, etc.
So I'm guessing you are talking about newly available equipment? As a hobbyist, I would consider some of the lower-end DC calibrators that might be available used for a few hundred US$ like the various EDC/Krohn-Hite calibrators, Analogic/Data Precision 8200, Valhalla 2701A/B/C/2707 and Keithley 230. Many of them have (optional) remote control via GPIB. Maybe a precision power supply like the HP/Agilent 6625A/6626A/6628A/6629A, would work? If you're lucky, you can find these for almost the cost of shipping. I think I paid US$ 20 (+ much more in shipping) for one.
Of the devices that are available new, the alternative that I can think of is the
IET VI-700. There is a GPIB option. It performs fine, but is quite clunky to use. It has a GPIB option, but the GPIB control is limited to the current range, it can't switch ranges via GPIB. If you have to pay IET prices, price/performance probably sucks.
Could you use a less accurate (but stable) source with a DMM connected that continuously monitors the voltage work?