What kind a hobby rocket or a high powered rocket?
I know that for hobby rockets at least you can buy parts like nosecones and fins. As for high powered rockets I don't know.
I personally found http://www.nakka-rocketry.net/ very usefull for this type of thing. It has lots of information on different fuels, motors, recovery systems, and the theory of the subject. It is definately a good starting point. I have used the information there for the design of what will hopefully be my first homemade rocket.