Keycaps are fairly straightforward to 3d print.

If that's not DIY enough you can make a mold and cast them from silicone (the method used by most artisan keycaps) or make them out of wood.

I have seen a parametric keycap library here https://github.com/rsheldiii/KeyV2

But standard FDM printing will not get you keycap that are smooth on the top, as they tend to be sculpted to fit a finger better, and you're still looking at ~0.4mm layers at best. You may be able to do it with SLA 3D printers which are able to do higher resolutions than FDM printers.

https://imgur.com/a/AalpV1r is an interesting project that 3d printed the keycaps too

There are a lot of people who 3D print keycaps I believe. I skipped that and bought DSA blanks from Signature Plastics.

Yeah, you can 3D print key-caps reasonably easily. Sorry, I can't remember the exact 3D model, but there's a free Cherry MX switch model around somewhere that surprisingly has the entire interior mechanism modeled. You can pull out the key mesh and tweak it a little bit to meet your needs, then it prints really well and fits nicely on Cherry (or Gateron) switches. There's also a handful of pre-created 3D printable keycaps on Thingiverse, however when I last looked into it I wasn't very happy with them.

Although, I must admit, whilst I've had all my components (including RGB LEDs and transparent Gateron switches) for what must be over a year now, I still haven't gotten around to finishing the project!