Unfortunately,lots of the stuff you find by searching the web is ripoff marketeer bs. They excel at peppering the web with references to their products and amazingly every one of them is #1 Rated, Most Popular, Best Buy, w/ testimonials and fake review sites etc bs
If you want something easy and good w/ prefilled and blank cartomizers available look at KR808D-1 models: Vapor4Life aka V4L or SmokelessImage/Bloogplanet/Halocigs G6 w/ manual batteries. Manuals because they are harder to kill with juice leaks and make it easier to get a good hit. These are the strongest slim ecigs. Not as slim as a cig because no decent ecig is slim as a cig. These are about 9.3mm diameter.
Vapor4Life - not rubberized, unregulated 3.7 (4.2 fresh off the charger), custom 10-13 cutoff allows very long drags, manuals still have a hole in the threaded end but it seems sort of sealed inside - not sure how well. I am a fan of the long cutoff because with manuals you press the button is 1-2 seconds early so good vapor is waiting when you start dragging so the 5 second cutoff other KR808D-1 models often cuts off my drag
SmokeslessImage/Bloogplanet/Halocigs G6 - rubberized finish, more colors, manuals are very well sealed, and by using regulated 3.6v (an even 3.6v) they get 40mah additional charge for each size. 5 second cutoff
I use both. I prefer the quality of the cartos of the rubberized models though my 2 main juices were found at V4L. I keep the cartomizers damp with eliquid to avoid singeing and I can get 7 days use out of each cartomizer. I use V4L WOW Juices, juices from the others I mentioned, and PG juices with 10-20% VG from elsewhere.