These days I go through the equivalent of 2-3 full cartos a day I would estimate (I switch among five different flavors and top them all up). I was a half-pack-a-day smoker but the TastyVapor flavors are just too dang good

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V4L is a fine supplier with many fans and I love their quick shipping, but I have to say their prefilled flavors don't quite agree with me -- that's just to illustrate that individual tastes will vary.
I would agree with Worutaa on getting juice from a couple of places, in sample sizes to start. (Myfreedomsmokes.com has a sale right now and they have 6ml bottles. Nhaler.com has great flavorful juice too, in 6ml sizes. Ms. T's Bakery and DIYflavorshack do too, they're both closed for the holidays temporarily. And TastyVapor.us doesn't have sample sizes, but that lets him keep prices rock-bottom. Vermont Vapor offers VG-only juice in samples too. and so on...)
Also get some blank cartos at some point and try filling yourself -- I find the prefilled just have a funny undertaste to them after a couple of refills and they don't last for as many fills. Any KR808d-1 blank cartomizers from any source are compatible with the Vapor King, as it is a branded KR808d-1.
(The best price on small quantities of blanks I have found is Mist-Stix.com with the "ECF" discount and First Class shipping. Best prices on large quantities, HealthCabin. If you're willing to wait for registered airmail to lower the shipping cost, Heaven Gifts is competitive too.)
Check out the KR808d-1 model subforum for tips too.
If you find you'd like to try a different model of atomizer sometime, you can buy KR8 adapters to the 510 or 801 (I believe KR8s are already compatible with 901 attys).