best location to find posi / locker for 8"

i cant afford to upgade to 9" or 8.8" at this point but I do want better traction than a single tire spin. where is the best place to go / look for a good used 8" limited slip / locker? also what would I be looking to spend on one.

I have beem salivating at 9" or 8.8 for a while but the reality of the situation is I can afford one that I really want for a while, and even getting one from a truck would require shortening etc... that all costs $$$ so I want to do it right the first time when I drop the dime.

thanks for any help!