If you search the forum you'll see quite a few posts on this topic. However -
New kegs cost upwards of $150 or so
This is what the breweries pay
Buying used (and hopefully legal decommissioned) you should expect to pay around $30-50 or more
Other means may not be as legal - as in buying on Craigslist (if you do, please be sure to go for the ones from the crappy breweries that make that colored water, you know the ones I'm talking about I'm sure)
I'd suggest you contact local breweries and liquor stores. If you can contact distributors in your area you might get lucky there as well. Good luck in your search