Wow, that's weird. One of the first articles i ever found about it was about it being used in cow feed. (Which i stumbled upon wen researching what exactly the U.S. feeds cows.) (DE makes for MANY less flies around dairy farms and such.)
We get ours from a tiny local farm supply store.
I suggest looking up a few brands and either writing to them or calling and asking where they distribute to. (I just looked and the first brand we bought was Glorious Gardens and the second brand was Concern.)
I found this link where they talk about where to find it as well as other aspects...
http://forums2.gardenweb.com/forums/...010216853.html