Here’s a cool site for prototyping and modeling techniques for design.
One thing is that you don’t need fancy tools like 3D printers for prototyping. You can go a long way with just simple tools and materials. Yes, a 3D printer makes wonderful shapes and stuff, but you can do the same thing with knives, balsa wood and clay.
Some more prototype techniques using cardboard.
Once you’ve scratched out your shape, even using cardboard as a master it’s not that difficult to add some fillers like auto putty, do some sanding and make a mold.
Once your do that, you are one step to an almost work alike, look alike prototype.
Some more on model making.
And other resources.
Along with my general book list.
The Let’s Build Series.