If you want good 3ohm cartos, I highly suggest boge sr cartos. Best price I've found (outside china, with crazy shipping and longer wait times) is at
www.crystalclearvaping.com in their 15 carto deals. The boge lr 2.0ohm will work great on the lava or tox also, but you won't be able to get all the way up to 6, by a long shot. I also like the 1.7 resurrectors... they're low resistance, xl size, so they fit in all the dc tanks, but they're single coil, and I love the way they perform. Again, won't be able to get waaay high on the voltage, but here's the thing... you won't need to. Lr stuff gives the impression of a higher voltage vape. So a lr single coil will feel similar to 5v at 4v. That's my experience, anyway. All three of those are in the 15 carto deals at ccv, as well as the dual coils. Now from the reviews I've seen on dual coils w lava tube, those really don't get up far enough, just because of the 2.5amp restriction. I just this second got done ordering tox, after about 5 days of nonstop research one em. I can't afford the provari either, and the tox will be my main vape. Very best thing you can do for yourself if you're considering tox, is watch the PhilBusardo lavatube review. Its about 35 mins long, but thats for a reason. He takes cartos at a ton of different resistances, and measures what voltage the lt cuts you off at. So you'll be able to tell if a lava tube/ or a tox, which basically is supposed to perform the same way, with lots of updated cosmetic/size features will work well with the way you vape.
I can tell you I'm really looking forward to mine, because I've been using a vv boxmod for last 6 mos (to try out vv and see if I need it) and I've discovered that a 1.7 resurrector (single coil) at about a 3.9 or 4.0 is an amazing vape... as is a 2.0 lr boge at about 4.25 or a 3.0 ohm bog at about a 4.5v... and variable voltage is the only way to get there. LT or Tox both let you get to those levels, or close to it, so I'll be happy.
But its my thinking that anyone with a carto preference that is considering lava or tox (and I keep comparing the two because they have VERY similar restrictions and safety features... only tox is supposed to be slightly better because it very quickly gradually revs up from 0v to whatever you ask for... within the 2.5 amp restriction... so its a tiny bit friendlier to your cartos and attys) should take the time to watch that very detailed lava tube review, and find out if it'll work within your style of vaping. You can find it on Phil Busardo's site..
TasteYourJuice.com
Good grief.. wrote all that and didn't answer your very simple question... the answer to your tank problem is either a 2.0 or 3.0 boge (they're standard size) in most 3 ml tanks or a resurrector 1.7 (they're extra large, but single coil) which will fit in most 5.5ml or 6ml tanks like the smoktech ones. I've also seen XL Boges around, which I'm thinking will fit in the 5.5 or 6ml tanks, but I personally haven't tried that yet. Guessing someone around here has. But that xl boge in a 3ohm would get you a carto that works in big tank and is high enough resistance to hit those high voltages.
And Dougiestyle... I've tried dc, but never higher resistance ones. Can you recommend a good dc that's a higher resistance model? I just don't know where to begin. If you know offhand the best place to get em, I'd be thankful.