Here's where I got mine. I bought a black, white, and three colors. So far the black and translucent white have been most useful. The colors I thought might be useful but for backgrounds, it's just too much, most of the time. Maybe I'll find a subject that needs a nice shiny midnight blue background.
And yes, even the black reflects to some extent which can be useful or a distraction. I like it. For dull I have Duvatine.
http://www.usplastic.com/catalog/item.aspx?itemid=31886&catid=442White #2447 allows 35% light transmit - 1/4" 12" x 24" Acrylic $18.30 per sheet.
Once you start shopping around Lowe's, Menards, and places like that, you will probably find, that translucent white is much harder to find that it should be.
Also you can start to see it's not cheap for a good sheet of plastic. But at least you have a site to look and get some ideas.
If you have a local plastic distributor who sells to the public, you can pick it up. With gas what it is now a days, it might be cheaper paying for shipping.
I selected the 12x24 as the useful size. You may be able to do with 12x12 if you only shoot small things, or maybe need a 24x24 for a larger platform?