Go to Daniel Siebert's site. He's a good guy, fair with prices, a fellow ethnobotanist, and has one of the most varied collections of salvia divinorum products for sale that I've seen anywhere. SageWisdom.org, I believe.
If you're looking for completely outrageous concentrated strengths at prices that are less cutthroat than some other vendors, go to everybodydoesit.co.uk. I believe they even have some 60x there. Be careful when using something that strong.
~Ethno