I've ordered several times from YuccaDo and enjoyed healthy, beautiful agaves, manfredas, yuccas, and other kinds of very drought-tolerant plants. They're located in Giddings, TX.

From their newsletter:

"Our 2011 summer sale has begun. Click to see over 165 plants on sale, ranging from 10-50% off! Sale ends August 27th, though you can request plants to be shipped in the fall (prior to the end of September). NO SHIPPING August!"

YuccaDo sale