Switch uses size Y00 according to
https://www.ifixit.com/Teardown/Nintendo+Switch+Teardown/78263
And I had the same problem as you. Even the screwdrivers that came with my replacement JoyCon shells weren't a perfect fit, and ended up stripping a couple of the screws. Turns out it was size Y0 and not Y00. It's not easy to find Y00, the Chinese sellers only mention the diameter in the listing. So I ended up getting an iFixit Pro Tech Toolkit and their Y00 bit is a perfect fit. It was $60 but I will probably never need to buy another screwdriver again, as it comes with a lifetime warranty.
They also sell just a Y00 bit for $2.99 (linked in the teardown above)
Their tools are much higher quality than the crap from China so you won't have to worry about it wearing out any time soon or stripping your screws.
But if you have the money for it and you might be taking apart other stuff in the future, the Pro Tech Toolkit is IMO a great purchase. Saved me a lot of frustration.
