LiteCig and GoodProphets, both in the state of PA carry prefilled 510 cartridges and cartomizers, I just don't remember which one carries which.
Between the two, which is what I meant, you can get 510 prefilled refills.
You're a supplier yourself though, you don't carry them?
Prefills I really don't use, but I just happened to remember that they had them listed on their websites.