To get the most utility out of your 3d printer, you should learn how to use cad (computer aided design) software. Cad software will allow you to design custom parts that you cannot find online.
When I got my 3d printer I wanted to print a replacement keyfob cover for my car key. The original one was heavy, bulky, and would easily press the keys when in my pocket. Leading me to come back to an unlocked car. After looking around online to see if anyone has done the same, I found a couple of available models but they weren't what I had in mind. So I set out to design my own.
While looking for different cad software, I settled on solvespace. It is an open source program that just works, is free, and is available on linux macos windows. I have been using linux for about five years now, so finding good cad software on linux was a must. I also didn't want to pay for any of the other well known cad software because they are not on linux, and many of them are moving to a subscription model. I'm not going to pay all this money for cad software so I can make some simple models every once in a while. There are some other open source cad projects that work on linux. Testing them ended in frustration because most of the time they wouldn't work as described in the documentation. I watched a single tutorial on solvespace from a man on youtube named Paul Lutus titled: Learn 3D Design with SolveSpace. With this tutorial and my geometry knowledge I haven't used since highschool, I was able to complete my keyfob project.
Website: https://solvespace.com
Github: https://github.com/solvespace/solvespace
Tutorial Video: https://www.youtube.com/watch?v=IlY1YFid8HA
Solvespace is uncluttered and just works. The learning curve to start making simple objects is very small. If you want to start learning 3d design to make prints without being tied to the ever-changing pricing model of commercial cad software, I recommend you start with solvespace.
Below are the files to the keyfob project. I have included the solvespace file and the stl files. The top is printed in tpu, the bottom is printed in pla. The two parts are glued together. This cover fits the subaru key fob.