A 3D Printer and CAD software, especially if you can get around free software.
Break a plastic thingy, why spend $50 on a replacement when you can make one. On no that broke, learn why and make it better.
I’m at the point where I can 3D print small tablet cases, and it’s funny watching the included injection moulded accessories fall apart, while mine is going strong.
It’s not for everyone, and there is a skill gap that’s bigger than most people are comfortable jumping. But if you have the desire/want to learn CAD or 3D printing, it will pay for itself, if you use it right.
Depends on what it is. There are no "Blueprints" online (as much as I wish there was), but there are repositories of 3D Printable models, like Thingiverse from Ultimaker and Printables from Prusa Research. For example if you wanted a replacement Stylus for your Nintendo DS or 3DS you can download them for free. For really popular things (or things nerds love), you'll find a model
However as the idiot who made the 3DS stylus, I had to make the models myself with a pair of caliper and dozens of test prints. It takes time and patience but the effort is usually worth it since the next time it much less and reduces the more you make.
Fusion 360 or Tinker CAD are good starting software, with FreeCAD, or OpenSCAD as alternative. With Blender if you prefer modeling like clay.
Ender3S1 is a Good Starter printer for cheap, with Bamboo Lab and Prusa being the go to community printers. My preference is Ender and Prusa since there are replacement parts easily available.