The beer keg, when properly designed, can easily become a 15-gallon boil pot or fermentor. But, because they are used extensively by beer-brewing hobbyists, used beer kegs can be difficult to find. It doesn't help that it is illegal to re-use kegs from breweries or convenience stores that sell full kegs for consumption. Unless you live near a factory that produces kegs, your best option to buy a used one is likely online.
Determine what size used beer keg you want. Typical keg sizes are 5, 7.5, 15 and 15.5 gallons. Five-gallon kegs are easy to find and can be purchased at any home-brewing store (a link for 5-gallon kegs is provided in Resources). Five-gallon kegs are too small to be crafted as actual brewing equipment--but if you just want to keep beer on tap, they work. If you want larger used kegs, such as quarter- and half-barrel-sized, you will need to look more extensively.
The second link in the Resources section takes you to sites that carry quarter- and half-barrel kegs (7.5 and 15 gallons). The listings periodically show as "sold out." If this is the case, check back at the site often. If you find a product you like, decide your quantity and purchase the product. Look for other websites that sell used kegs. Generally speaking, a used keg can run anywhere from $60-$100. It all depends on the quality of the used keg and who is selling it.
Search websites like Craigslist.com and eBay.com. These sites occasionally have used beer kegs listed. If you see one, however, buy it quickly because they tend to sell fast. The "Buy Now" feature on eBay may be the best way to secure a used beer keg before someone else does.
For those who do live close to a factory, call and ask for any used kegs available for sale. These will be kegs with dents or other nicks that prevent them from being sold to distributors. These dents and nicks are not dangerous, however, and will not cause any pressure issues.
Do not purchase a full keg from a store, consume the contents, then keep the empty keg. This is illegal.