As others have said, Thule and Yakima are the industry standards.
My advice would be to check out eBay and Craigslist to find what you need. New set-ups are pretty damn expensive and used ones can be found pretty cheap. If you go with the Yakima, as stated above, the only thing specific you need is the Q-clips.
If you choose to piece together your own kit from used parts (as I am in the process of doing), you'll need 4 Q-towers, 2 cross-bars (probably 48'' but your Sonata could be a little larger than I'm thinking), 4 Q-clips (think you need the Q39s), and one large ski rack or two small ones, which can usually be bought cheaper than a single large one. Hope that helps!