Welcome to the site. KFX is the only site that I know of that has performance parts for it. And about repacing the stock radio when I replaced mine in my Sorento I went to crutchfield.com, typed in my year, make and model and up came the headunits that would fit in it. Many choices and a large price range. I personally like Pioneer but there are also many other good brands out there. Enjoy your stay here.
You can put in any deck you'd like - there are simple connectors you can buy for $10 that let you use the factory wiring harness. I just put a nice Sony deck with XM tuner in my Spectra5, I keep meaning to post pics of it here... The silver bezel that goes around the whole dash console is a real b**ch to get out.... there are six clips (three on each side) that hold it in, and it's REALLY tight. Use something plastic to pry it out with or you will damage the finish, and once you have it open it's got so much room to do pretty much anything you'd ever want in there. I get the feeling they figured people would want to replace the whole console with custom ones, because they sure made it simple to replace.....
Pinchy's right, you can put most any deck in there of your brand preference. Shop around though definitely, surprisingly, even local specialty audio shops carry the same products you can see online with cheaper price tags. Also, do your homework. Check out the various features of the head units and see which ones you can make use of and which you won't. For instance, there's no use in buying a head with a built in xm tuner if you never intend to use it. Stuff like that. It'll save you from wanting to drop a rather good chunk of change down the road. I've got an alpine head unit and am fairly satisfied with it!
OH and just to re-iterate what pinchy said. BUY THE HARNESS! :grin: It's the easiest route both for install and for if you want to change units down the road.