You can get a good "starter kit" for buck riveting from Vintage Trailer Supply (or you can order the pieces ala-carte as well). The kit has a buck riveting gun a rivet cutter, 25 or so clecos for 5/32" riveting, a bucking bar, etc.. Depending on how much riveting you intend to do, you may want to order at least 50 of the 5/32" clecos, and at least 25 of the 1/8" clecos for holding things in place that will be pop-riveted with 1/8" rivets.
Airstream Buck Riveting Kit
You can also get tools from The Yard store inc., Aircraft Spruce, and Airparts, Inc., if you want to do some comparison shopping and hunt around. I bought the VTS kit and have been very satisfied with it.