where? well... anywhere that sells computer parts!!
To give an oppinion on what kind of upgrade you want please let us know your system specs and what games, and at what resolutions, you play.
Depending on where you are from Newegg.com/ca is always a good bet
Though Canadacomputers.com(eastern canada) and NCIX.com(western) generally have better pricing and more customizable shipping if you live in canada, though neweg cannot be competed with in terms of optons.