I found two solutions.

The first is a 12V powered selectable voltage converter. It was 16 bucks and included a 120AC-12DC converter for home use. Here are a couple of pics...

I bought it at batteryspace.com, but I saw a similar one at my local Rite-Aid, so I'd say they're available just about anywhere. This was the only, however, that I saw come with a 120AC adapter.

The 6V voltage is 6.14, which should be good enough for accurate operation. I power it with my Jump-N-Carry jump-start/power supply unit. Weather report says I won't see stars for a full week from today so that's when I'll check how well it tracks.

Not content to leave well enough alone, I also purchased a NiMH battery and tiny voltage regulator that I intend to double-stick tape to the back of the drive controller. That would make one less wire to trip over. I'll let you guys know how it goes. Here's a pic of the parts...