There are a lot of places to buy kegs, but prices and availability have been greatly impacted the last few years.
There are two types of corny kegs that brewers use. There are ball lock which were used by the likes a Pepsi and there are pin locks, which were used by the likes of Coca Cola. The main differences between these two are:
Ball locks - use a ball lock type system on the Corny Quick Disconnects. These are slightly taller (23") and slightly smaller diameter (9") than pin locks.
Also, generally speaking, ball locks have been preferred by brewers because they have a pressure relief valve. Aside from these differences, they function pretty much the same. Ball locks currently run in the $60.00 per keg range while the pin locks are a bit cheaper. The problem is that the soft drink makers are not using these as much now, and both pin locks and ball locks are become more scarce, and therefore more expensive.
If you are not committed to either style of connection, you might pick some up from
Adventures in Home Brewing. I purchased mine there a few years ago because they had some pretty good prices. Sometimes they will offer specials on four-packs of kegs.
There are other places to buy, but that is where I got mine.
Finally, if you are absolutely set on ball locks, there are after market kits that you can use to change out the pin lock posts for ball lock posts.
I hope that this helps.
Mark