Craigslist or any of the online garage sale type pages on social media are the cheapest. You get what you get, however.
Many online vendors sell them. I've bought 8 over the years, some from AIH, some from Kegconnection. I bought all of them, at various times, when there was a "blue light special" type of thing going on. The best deal I got was from AIH; 4 pin locks for $100. They came pressurized but not cleaned (still had a bit of soda in the bottom). The kegs I got from Kegconnection were pressurized, clean and had new o-rings installed, for only a few dollars more.
If you want them yesterday just check out all the online sources and go with the best price you can find. Used kegs are pretty much used kegs. You're going to want to disassemble, inspect, clean, and replace the small parts regardless of who you buy them from.