It depends on what town you're in, but usually you'd buy them at where the derby's going to be. Some of them have web sites that allow pre-purchase, I know many of the events in the US use BrownPaperTickets.com for pre-sales.
Greensboro Roller Derby was created in 2010.
Roller Derby - brand - was created in 1936.
Roller Derby Germany was created in 2011.
Helsinki Roller Derby was created in 2009.
Houston Roller Derby was created in 2005.
Pacific Roller Derby was created in 2008.
Junior roller derby was created in 2007.
Roller Derby Toulouse was created in 2010.
Wasatch Roller Derby was created in 2008.
NEO Roller Derby was created in 2007.
Toronto Roller Derby was created in 2006.
Maine Roller Derby was created in 2006.