IMHO, grow your own plant, and dont use extracts. The plant is very powerful on its own, there's much talk of reverse tolerance, and if its not working, you probably gotta adjust your smoking technique(most dont get the salvinorin absorbed into their body so they'll smoke 10 doses but have no effects).

for smoking salvia:

a)lots of heat. you can burn plant, but you need it pretty hot to release the salvinorin. A bic will work, but i prefer to torch it JUST incase

b) HOLD IT. it takes a while to absorb. with some experience, you'll figure out how much time you got to smoke before it kicks in.(you only need like 1-3 hits, better to hold than to take another hit)

c) SITTER -- very important. when first smoking salvia, you aren't sure when its going to hit you, so you may keep toking til it does. have the sitter be ready to take the pipe/bong whatever because it hits you instantly and you can drop it/drop a cherry and not be able to do anything about it til 15 min later. you may end up having a horrid burn, burning something, stepping on shattered bong glass, etc. once you have some experience, you'll know when you need to put it down