I've had excellent luck with the Smoktech DC tanks, there is a 3.5ml version that fits perfectly on an eGo, although you also should get the little thread protector/sleeve for the battery, as it looks like trash without one.
There are two versions (that I've seen at least) the ones with black plastic endcaps, and the aluminum endcaps. The black plastic endcaps are more functional, they have a slot that fits into the flange on the base of the carto so that your tank never spins on the carto. They also seal very well against the tank tube and the carto.
The Aluminum endcaps, in my experience, are less functional but far nicer looking. The bottom endcap does NOT have the slot milled into it like the plastic ones do, so your tank will spin freely on the carto. Makes it a hassle to screw down tightly. They also fit a bit looser over the carto, so it can move up and down on the carto (like pulling the tank off the carto when it's full of juice) I'm not sure if this was a production run issue or what, but it's just my experience. Even though it's a looser fit I have yet to experience any leakage whatsoever.
There are also some smaller 2.5ml (I think) Smoktech tanks that will look seamless on your eGo. I can't tell you anything about them as I've never tried them, just don't like the way they look. I forgot to mention, these come with a thread cover/sleeve built into the design so you don't need any additional stuff.