Tamkesh I had a similar problem in reverse, if that makes sense? Everything I have is 808 threaded and I needed, in an emergency, to use 510 threaded cartos. If you go to the homepage and look under adapters Vapor4Life is one of the few places that carries a 808 to 510 adapter, which is what I needed. They also carry the 510 to 808 adapter , which is the one it sounds like you need.
They're about $3.50 each and you only need one. For the adapters the first number denotes the the type battery, the second the thread of the carto . Take a look and see if something like that might help. Good luck!
Ms. Tamkash Vapor4Life only had the 510 cartos for a mod that they haven't carried in almost 2 years. They prefer to stick to 808, since all but 1 item they sell currently is 808 threading by default. Can pm you a few reputable places though....