The quake script is almost certainly a good (& FREE!) solution for what you're looking for. If you're looking to pick up a "package" of effects, New Blue FX is held in high esteem in this forum and their Motion Effects (either "Active Camera" or "Earthquake") package would suite you as well. Last year they had some smokin' deals for the "12 Days of Christmas" so check back on their site in a couple weeks if you can wait.