Though there are plenty of great internet-based dealers, I'm a huge fan of supporting the 'brick and mortar' shops. A lot of these places are facing difficult times simply due to the drastic ratio of online shoppers to brick-and-mortar shoppers. If you're lazy and don't want to leave your bedroom, or you simply don't have a magic shop within reasonable distance of your home, you can get the best of both worlds in many cases since most magic shops have websites where you can order the products online. So you're still keeping physical magic shops alive AND being lazy by buying online.
My personal favourite, and a shop I will continue to support always: Browser's Den of Magic in Toronto. I live in a different city but I've often made a two hour commute to the shop by taking the train, subway, and bus (all three, not just one or the other!). Thing is, with a lot of these shops you can easily stay for an entire day. There are usually guys sitting around jamming and the atmosphere is, at least in my experience, very relaxed.