You mean cartos as in cartomizers - the screw-on combined cartridge and atomizer of the 2-piece ecig?
You if you have a popular brand then you can buy blank cartomizers and fill them yourself. That's what I do. I own 510 and KR808D-1 (a Vapor4Life one) and blank cartomizers are sold for those models and a couple of others. 510 and KR808D-1 have the most variety of blank cartomizer types.