It really kind of depends on what supplies you're looking for. I go to several different vendors for different supplies. Bottles, syringes, flavoring, nicotine, pg and vg, etc. Some things it's easy to find vendors for, such as flavoring.
Everyone knows about the likes of Lorann's, The Perfumer's Apprentice, FlavorArt, etc. for flavorings.
For empty bottles, I tend to use litecigusa.net, but that's more because I love them so much and buy from them all the time. Hey, empty bottles gets me a point or two, so I might as well.
I get my nic from XtremeVaping.com. It's fair priced and high quality, as far as I can tell.
Other things I just get from my local pharmacy, such as VG, PG, and syringes.
Those are just the places that I go to. They've never done me wrong so I keep going back to them, but there are plenty of other suppliers out there that are just as good, I'm sure.