Most manufacturer cartridges use loong totem/e-cig.com liquid, or something as close as to be indistinguishable, anyway. It is harsh, and produces a lot of vapor. You can get liquid from e-cig.com, in up to 50ml bottles.
Janty is an exception, they use a different source, and it is much smoother, but produces a bit less vapor. They are just starting to sell it as a liquid, but only in 10ml bottles, IIRC. E-puffer and e-cig-sales.com also sell a liquid which if it isn't the same, is close enough to Janty's, and they have 30ml bottles.
Freesmoke and JC are the smoothest, with no harshness at all. I can still feel the kick, when I use the high strength, but I can understand how if you're used to the harshness, you might not notice the kick without it. Anyway, Freesmoke produces the least vapor of any liquid I've used. JC, OTOH, produces just a little less vapor than e-cig.com's.