Hi, I was in your shoes not long ago. I got a vtube from apolloecigs.com. It uses an 18650 for the full size one and it's variable volt from 3 to 6 volt. I love it and now I have a stainless mini as well. It uses an 18350 battery. For me, it lasts almost a full day before needing charged. The devices are pretty cheap, but the batteries and chargers add to the price.
I hope this helps. Message me if you have more questions.