Really you just need
juice. FreedomsmakeUSA has small samples and great service. Vaporite has organic and samples, but some juices take a week or two to steep before they taste good. My favorite juice is BackwoodsBrew.net, but his site is down until Monday. He sells 5ml bottles. Gourmet Vapor has decent flavors too.
If you can take the white mouthpiece top off your cartomizer with a paper clip, you can drip juice into it. Blank carts are best for a fresh start, but adding another flavor to a prefilled works too. You just have to vape through the mixed taste or learn to wash them.
What system do you have? Vapor4Life has a 30% off sale going until the 30th for blank KR808D1s. SuperSale30 for >$60 orders. I have never tried their juices, but they have tons of flavors.