Craigslist. Although every used car I bought came with them already installed.
It's like everything else--you gotta run the numbers. What does a new set cost, between purchase, mount, balance? How long will they last? Now compare against what a used set will cost, then what will mount & balance cost. And how long will they last? Compare the cost per mile.
I suspect that "most" of us who go down the used path are gaming the system by doing our own mount and balance. And are willing to put up with the occasional bad tire (which would be money lost). IMO, most people would be better buying new and getting the long life with some sort of road hazard warranty for the slim chance of needing said warranty. "Big" money up front, but divided out over the long life of the tire, they are probably hitting the point of both low cost and low effort.
Just take into account how much you drive. If you are doing only 5k/year then you probably don't need high end tires, as they will likely age out before wearing out.
Also, if you do go down the used path, make sure you know how to measure tread depth, how to look for tire issues, and how to read the date code (so as to avoid buying old tires).
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I took a gamble on a used set. $100 for a set of nearly new tires--someone traded in a car at some used car lot, the lot owner apparently didn't care so sold them cheap on CL. I snagged them, mounted them myself, paid a local shop $10 to balance all 4. $110 for a set? I only got 36,000 miles out of them but that was darn close the 40k I normally get. I haven't done that again as it was pure luck on that deal. I also do lots of high speed highway driving and so... I sleep better with "quality" tires. YMMV.