Go to the Games Workshop online store. Start digging through the models available for your army. Models that have bitz available will have a "bitz" button under their picture. This will pull up pictures of individual bitz and sprues.
They also have a really good search engine if you are looking for specific parts. Remember to use quotation marks. Entering missle launcher will pull up every model that has either the word missle or the word launcher in it's name. You get a bunch of models you don't want to have to sift through. Entering "missle launcher" with the quotation marks will pull up only the models that have BOTH missle and launcher in their names.