I'd recommend that you get these ones, but you can pick them up plenty of places that aren't Amazon too, just make certain you get the ones made by Gulikit, any others are probably as cheap as your originals and you'll have to come back and replace them again in short order. The Gulikit ones use Hall Effect sensing rather than resistive contact pads that will eventually scrape down and break.

That kit at Amazon comes with all the tools to do the job and as the sticks are Hall Effect based, they'll theoretically never drift unlike the ones that ship with Joy-Cons straight from Nintendo.

iFixIt has the process for doing the work: https://www.ifixit.com/Guide/Left+Joy-Con+Joystick+Replacement/113182 and https://www.ifixit.com/Guide/Right+Joy-Con+Joystick+Replacement/113185