[Test-Equipment] rms voltmeter?

[David DiGiacomo]

>Anyone know what sort of "standard" one usually uses to calibrate an "AC" 
>indicating instrument?

An AC voltage calibrator such as an HP 6920, Fluke 515A, Fluke 760A,
Fluke 5100, or many others.

At higher frequencies, an oscilloscope calibrator/constant amplitude
signal generator such as the Tek 191 or SG503.

>It's much easier to find a DC voltage source that's accurate.  Where does 
>one find an "accurate" (1% MAX or tighter)?

Again, there are many calibrators available, such as the units above, and
also DC only units such as the Fluke 332.

Or, you can find an old fashioned standard cell, often available very
cheaply, but a bit hard to use.

Or, you can build a standard out of one of many precalibrated IC voltage
reference ICs that are now available.  This has been discussed to death,
so I suggest you try a web search.