If you are worried about the wallwart changing voltages based on load, you can get an AC->AC transformer, then build your own adjustable voltage power supply using the same LM317's daedal uses with his driver in voltage regulating mode rather than current mode. That's what I'm doing right now. It should be much more accurate and stable than a generic wallwart, since most of them are indeed crappy. You would also want a current regulating driver like daedal's since you would probably run the full 1.5A that the LM317 can do without one. Not as easy as just plugging in the thing and hooking up a couple wires, but probably much better for a diode.
American Science and Surplus has a nice 120VAC->24VAC transformer (link, #35651)
, then I used a rectifier from Radio Shack (link)
to convert to DC. Then you want to add in some caps for smoothing and then the LM317 circuit. There are lots of places on the internet that can give you drawings/tutorials on how roll your own power supply.
You can fit it into a small box and have a dial on the front that turns the pot until you have your desired voltage. I'm also thinking about getting a $3 meter (link)
and gutting it to have a voltage display built in
Also, I don't know about ordering online from AS&S, I am fortunate to live close to one of their three stores :P They also have first surface mirrors and the acrylic mirrors that AP_Digital_Light uses in his DIY FS Mirror guide. Hell, it is a random crap store. They stock random crap. Best store in the world.