Rives Audio makes a very good one (it has two sets of test tones- one flat, and one designed to compensate for the known deviations from flat of the popular Rat Shack analog spl meter), as does Stereophile Magazine. Rives you can get from their own site for about $20 (if I recall- I bought mine years ago), and the Stereophile ones can be purchased from the mag website or from Parts Express.
Just be sure you get the meter, too. One day I'd like something a bit more sophisticated, but the Rat Shack will do a fine job.