The following is the text of an email from a grad student here at UNC. He reports his findings:

After doing a bit of research, it seems what one typically uses when trying to acquire reflectance measurements is something called a "diffuse reflectance standard". These are made out of materials which have very high reflectance (~98%) and which are very even across the spectrum. The best material available today seems to be a plastic-like material called PTFE. Second to that is Magnesium Oxide.

In our measurements, we used white paper with an unknown spectral response as our standard to compute reflectances, but it's pretty clear that's not a good standard (one of our paints was actually whiter than our standard!) and probably doesn't have very even reflectance across the spectrum. Anyway, it seems like something that would be useful to have around to use in conjunction with our fancy SpectraScan PR715 photometer. The photometer can measure energy output from a surface but it can't tell you energy input, to get a measurement of that you need a white reference.

Anyway, here are some links to companies that sell standards:
(Note from webmaster: These links may or may not be current...)

I've sent out inquiries to these places. The one that does list a price wants between $300 and $400 for one.