No such thing as a dumb question. The choice of rivet will depend on what your doing. I use aluminum rivets and a hand rivet gun. The size of rivet may vary depending on the application. If you have a Fastenal store in your area, they have good prices and large selection.
McMaster-Carr is the best prices I have found. There is a link to then on the material/ mods thread. Rivet gun I use is a hand riveter called the Big Daddy. Works great.
I used aluminum rivets and a hand rivet gun to put together all of my decking. I got the Rivets from Fastenal. I think it was a box of 500 for a pretty good price. Fastenal is the way to go if they are in your area. I got all my self tapping screws from there too.