No clue where you can buy those, but you can make them really easily. Just get a little propane torch and heat a sheet of acrylic and fold it over a piece of wood. Glue on legs et voila!
Edit: Not sure if it would get hot enough, but a heat gun might also work.
You might try a place like WR Displays. I don't see anything on their website that's exactly like what you're looking for, but in my experience they sometimes carry stuff that's not listed on their website. Give them a call and tell them what you're looking for.
Alternately, as kybosh says, it's pretty easy to do that yourself if you're comfortable with a propane torch. I've bent acrylic before with a butane kitchen torch. Here's a pretty good tutorial:
Acrylic Riser Displays
