Salt, I recommend you start with the Turbos (especially if you want 555). Ben has them for 9 bucks and change, some discount for quantity (the more you buy, the more you save). I don't know if they are soft of hardcaps, but they are really good. Just a warning: the ones I got were seriously underfilled, so add some juice even before you start using them. And make sure the airflow tube is centered; if it's not, you'll burn in no time flat. And he offers first class shipping option.
I also like the V4L premium CoolCarts (blanks only, the prefilled 555 are NOT the new ones.)
I just placed an order with wordupecig. They have k808 cartos that have the same construction as the 4081s, and their shipping is free, so it's worth a try. Of course, I know you had no luck with SLB 510, which are supposed to be identical inside, so there.
You should get a sample Turbo from me tomorrow (555), so if you can wait, you'll be able to try if before you order.
Good luck.