Brian -
Wouldn't swear to it, but thought I saw it in Wal-Mart store when I was picking up toothpaste few months ago. Know for a fact saw it at a locally-owned (non-chain) drugstore awhile back, so you might check a "mom & pop" pharmacy/drugstore in your area.
My dad used toothpowder exclusively everyday of his toothbrushing-life. Swore by it.
By Groo's comment ? "never even seen the stuff" ? I'm guessing those of us familiar with the product are showing our age. <img src="/images/graemlins/frown.gif" alt="" />
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"Things that have never happened before happen all the time." — Scott Sagan, The Limits of Safety