Depends on how much money you want to spend? Do you want a new kart chassis or a used chassis? Do you want local track support or national team support? I personally when I got I to karting bought a kart off of Craigslist and it was the biggest mistake I made. I got totally ripped off and was sold a piece of crap and paid way to much for what I got.
I then bought a three race team Kosmic kart from J3 Conpetition for a huge discount and have never looked back. In fact I just sold my Kismic Kart and bought their new COMPKART and it is one of the best karts I have driven. Very easy to tune and fast and it looks awesome!
My point is I bought two karts from them because of their reputation as being a top notch race team with unparalleled customer support which is why I cont to do business with them. So my advice is to find a shop in your area or national race team and start building a relationship with them so you can their tech support and anything else that You may need. But again it all comes down to how much do you want to spend