Wherever I can get them cheap. MyFreedomSmokes.com was just running a good deal... a Joye Type B (normal resistance) with 5 tanks for $9.95... you just can't beat that, so I ordered 5. They are also the cheapest on Type Bs, normal or low res are typically $8.95, though I hear there may be a price hike shortly... They also sell the silicon caps usually, but I don't know if they're in stock or not. Most places that sell eGo-t stuff will have them, and I think they're going to become standard issue on the tanks sold. They're much easier to put on and take off, and they seal better... it's really a win-win.
Glad you found what you needed. Let me know if I can help...