Do you want to buy a used kayak or a new one?
If you are after a used one then there will be tons of old models on craigslist and what not. Also, check with CKS to see if they know of any used or consignment boats. (Not to mention people on here will have some).
As for new models, you would fit pretty well in a Jackson Star or 2Fun. The Star will be a little more playful where as the 2Fun will still let you play but also give you a little more stability on the river. Both boats have great outfitting that is easy to use and adjust. If you have any questions just ask me or check out www.jacksonkayak.com
If you want to look at other companies, Dagger, Wave Sport, Riot, etc will all have models in sizes that will work for you.
Just my own personal bias, don't start off in a "creek boat" (the big round ones). Although many people support these boats for beginners because they are so forgiving, I find that they teach bad habits in learning paddlers. Also, these boats are made for class IV and V paddling. After your initial learning phase, a creek boat will prevent you from playing around on most class II to III+ runs.
Basically, go with a boat that is within the range of 42-48 gallons in my opinion. Any company's website will have the specs on all of their boats including volume.