You can occasionally find kegs at special grocery stores. Also, since many brewers make root beer, too, they can keg some root beer on request. Good luck - if you find it, please post a reply telling everyone where you found it.
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Bottoms up!
Kevin Guske

So heres a tribute toast with root beer in hand to you and the many mugs of suds along your happy trails. --Charles Wysocki, artist and root beer fan, 1928-2002

I'm not sure you can have a keg mailed - they're pretty heavy so shipping would be outrageous. You might check with a local brewer / brew pub to see if they could provide one...

most liquor stores in my area (NJ) can order one for you on their usual alcohol order. i have gotten a few kegs of A Treat birch beer which was crazy good. but almost every store i called could get them. try that.

-anthony
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anthony's root beer barrel http://rootbeerbarrel.com