SakuraRetroModding for the custom buttons on etsy.
On amazon you can get some gullikit hall effect joysticks that won't ever have any drift.

"Gulikit Switch, Hall ThumbStick". Incapable of drift. Also uses slightly less power than the standard ones so battery life is a bit longer. Not too difficult but you do have to be careful with ribbon cables.

About $30 USD for 2. If you want to read about them and how they work you can read more from the manufacturer. https://www.gulikit.com/productinfo/945307.html

And if you want to buy them. Link below. Would recommend watching some tutorials on youtube regarding installation.

https://www.amazon.com/AKNES-Replacement-ThumbStick-Controller-Thumbstick/dp/B0BPS1R5D8

It's kind of a diy, but assuming you want just better joycons then you can pick up some hall effect joysticks from Gulikit (actually not too bad to do it). They're really good, have a much smaller dead zone, and are unlikely to be affected by drift. After that you could get some grips to make the joycons feel better.

Otherwise, you have things like the pro controller, 8-bitdo, etc.