For mods, I either use My Vapor Store or buy directly from the manufacturer.
For tanks or atomizers, again I use My Vapor Store, Fast Tech, or Eciggity.
For batteries I use RTD Vapor or IMR Batteries.
E-liquids, I'm all over the map (USA). I generally only buy specific commercial flavors. I haven't found a commercially prepared e-juice from any manufacturer where I enjoy more than a couple of flavors.
I also dabble in DIY mixing; I like Bull City Vapor for flavoring agents and Wizzard Labs for their concentrated nic, propylene glycol, and vegetable glycerin.