The best juice is rarely found in the same place as the best hardware.
For cartomizers, the 1.7ohm Smoketech Ressurectors and the 2ohm Boges are the best on a kGo, IMO.
The kGo can be bought from a lot of good, reliable vendors. The prices are all pretty close, but I'm sure there are some Fathers' day sales and coupon codes going on. The main differences are the color choices and the accessories they include. I would suggest getting a kit with atomizers. There are a couple reasons for this.
First, atomizers cost more than cartomizers and you need at least one if you're going to be trying different juices. You don't want to waste cartomizers testing juice because you can't really mix flavors in a cartomizer. If you get a kit with atomizers, you will have them for testing juice, and you won't have laid out as much money for them as if you had bought them separately.
Secondly, the cartomizers that come in kits are sometimes not as good quality as you can buy separately. So, if you get the kit with cartomizers, you're trading $16 worth of atomizers for $7 worth of questionable cartomizers. Bad deal.
CrystalClearVaping.com has a cartomizer "bundle". You can get 3 boxes of cartos for $20 shipped, and they'll throw in a free drip tip or two as well. OR, you can go to Surevapes.com and get 5 boxes for $23 shipped.
Good kGo vendors include:
Sweet-Vapes.com
Hoosierecigsupply.com
EliteVaporworks.com
DarkCityVapers.com
For juice, you're on your own. There are a million good juice vendors and everyone has different tastes. Juice is the biggest challenge to find one that suits you. I'd suggest getting small 5ml or 10ml bottles, or sample packs, from various juice vendors. Don't get larger bottles until you find one that you like. The chances are that you'll try two or three or more for every one you like well enough to buy a large bottle of. Any of the vendors here make good juice. If they didn't, they'd already have been crucified by the people here. You can find juice
reviews in other sections of this board, but take them at face value. It's really a very individual thing.