Sourcing empty 15gal (1/2bbl) beer kegs?

You can get new ones for $90 (new sanke keg link), or you can buy pre-converted stuff at brew magic for more money, of course. Recent Craig's List prices in NC for kegs with questionable heritage has been $30 to $50. The story behind these kegs was that the brewery rep was repeatedly called to pick-up the kegs, and the bar was out of space, so they went to whoever showed-up with the deposit money.

A keg weighs just over 30lbs, so keep in mind it can have a good chunk of value simply as stainless scrap (at $1.5/lb it's worth $50). This will set a floor price.

Making friends at a friendly neighbourhood microbrewery, if you are fortunate enough to have one, and buying one of their no-longer-serviceable kegs is one good way to go.

I bought one from the local beer store for the price of the keg deposit ($50). They often have them.

Craigslist is probably your best bet for finding used kegs. The ethics on it aren't exactly pristine (kegs are owned by the brewery and only "leased" to the consumer, usually with a deposit to ensure their return) but you can usually find ones where people have already lost their deposit on for the deposit cost or less.

I believe the $30-40 figure is when a brewery owns their own kegs, and wants to dispose of non-functional (by serving standards) kegs at near scrap cost. Ask your local breweries about their keg supply.

If the kegs are owned by a 3rd party, try them. For instance, it seems like Microstar owns most of the kegs around here, so I called them; they send them to Adventures in Homebrewing (homebrewing.org), who outfits them as mash/brew kettles; I'm getting another one delivered tomorrow, in fact.