I'm not sure which model you have. I checked a compatibility chart and found a "Cig O2" listed that may be it, may just be a typo in the chart. The Cig 02 is listed as a KR808-D. Hopefully someone that knows more than I do will confirm what model you have. The KR808 is a very popular model and there's a ton of information on it in this forum. It is listed as 110mm long (slightly longer than a 100 cig) and is a two piece like the one shown on the Cig2O web page. Once you find out for sure which model you have, you can order parts from many different vendors. Your cartomizers should be refillable as well with eliquid, giving better flavor than most prefilled cartos and saving you some money as well. The "cartridge" you mentioned, by the way, is called a cartomizer or carto for short. If you call it a cartridge on the forum, folks may think it is part of a three piece unit which you don't have. The Cigo2 description actually uses both terms.
Wish I could be more help. You might post some more details about your ecig to make it easier to identify.