Try searching the local Craigslist if you are near a major industrial area. They are also commonly found on Ebay.
A used 12x36 Atlas/Craftsman lathe in good condition would cost from about US$600 bare bones to more than twice that fully tooled. Bargains can be found anywhere in those ranges.
Your best chance of finding a suitable machine, fully tooled, and in good condition is to find a hobbyist user that is selling off his equipment. It's more likely to be kept in good condition and not abused.
There are cautions in buying a used American made lathe. For one thing, Atlas hasn't made them in over 30 years. For another, condition is everything in buying a used machine. You need to study up on what points to inspect in buying used machinery, or bring along a good machinist friend.
An Atlas lathe, used within it's limitations, can do very acceptable barreling and action work comparable to machines costing 3 and 4 times what you would spend on an Atlas..