Vote Smart has a long list of interest groups. When you click on the interest group, if they have them (and the small ones don't), you can see their report cards. With the NRA in the news after its press conference yesterday, you might want to show your students who each of its legislators have in terms of NRA ratings. The link is very comprehensive and includes not only federal level legislatures, but also state and local ones. It also includes links to the legislation the NRA used to judge the officials.