To be honest I think you should save your money until you can afford to pay for new ones. I don't know of anywhere you can get "gently used" tires (scratch that, see below). Tons of people sell used tires on Craigslist, but even the best I've seen promise minimum 50% tread but nothing more than that. You're also gonna need to look at the date of manufacture. You don't want old tires, even it they're brand new "old" tires. I wouldn't want to put 5 year old used tires on my car.
If they're an OEM size, you may be able to find some stock OEM take-offs on this forum. Some people swap out OEM tires almost immediately...because sometimes they suck.