eBay is always good. Different cards suit different people I guess, so it's hard to say. At some point though it's worth checking out a number of different cards, get one of each or something whenever you can. The most recommended ones, and which are all worth at least trying out in my opinion, are:
Old Style Studs
Aladdin 1001
Arrcos
Wynns
All E and T11 decks - Smoke and Mirror being my deck of choice, Centurions, Guardians, Propaganda, Ghost, Black Ghost, Black Tiger, Shadow Masters, Masterss 1800s, Vipers.