Ed look here : http://www.stouffer.net/Productlist.htm
Stouffer makes an assortment of reflection guides, even calibrated if need be. I don't believe Stouffer sells direct but I'll bet you can get them here: http://www.pitmanphotosupply.com/
where I used to get all sorts of production stuff including small guides from Pitman back when I was in the newspaper biz. Or probably B&H could get something.
My transmission step wedge ran away and hid so I'm shopping for a similar calibration standard too. A cheap but accurate one I hope.