When I used prefilled cartos from Vapor4Life or elsewhere (for my KR808D-1) I keep them damp with
juice and get as much as a week's use out of each.
I can use any thinner juices I want (I find thin clear juices keep the carto working well longer) and there are 100's of flavors. There are a couple of vendors alone that each have over 100 flavors. I generally like 80pg/20vg juices for cartomizers.
I use manual batteries (much safer from being damaged or killed by a juice leak - and when you keep cartos damp, you will occasionally get leaks) and I only use soft-cap cartos with a defined center air channel (they work well longer for me)
It is $2-3 cheaper per pack if I get blanks.
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You have KR808D-2 aka E9 - not as much stuff available for that model vs KR808D-1. However you can find some parts at AZSmokeFree as well as some prefilled cartos which may be lower quality Chinese juice and some blank cartos, blank cartomizers at Vaporkings.com, D2 cartos at LeCig.com as well as D1 to D2 and D2 to D1 adapters.
Other KR808D-2 models are Smoke 51 Duo, Green Smoke, E9, LeCig D2, Red Dragon, Vapure and I think South Beach Smoke. Some sites list Bloog as KR808D-2 but the current Bloog is KR808D-1
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KR808D-1 hardware and cartos can be found at Vapor4Life.com, SmokelessImage (Volt), HaloCigs (G6), Bloogplanet (MaxxFusion)
If you are rich and can afford to use cartos once and toss them, and you are happy with the flavors you have, then there is no reason for you to try filling.
Note - the 3-piece ecigs that come with atomizer and cart - those are also filled but that's an annoying way to vape. People who buy popular models of those switch to using blank cartomizers which hold more juice.