Which rivets and where?
It matters
McMaster Carr has a good rivet selection.
The steel rivets are set cold. I think most body rivets are 1/4 or smaller.
There are hand set rivet tools that you strike with a hammer but a pneumatic riveter is best.
A 3x gun will do the 3/16 and smaller steel rivets in the floor pans. & a 7x gun for larger steel rivets up to 1/4.
I’ve found a selection of used guns on eBay, 3x generally under $100 and the larger ones a bit more.
If you are not concerned with originality there are fasteners that look like rivet heads.