The easiest and surest way is to remove the bearings first and just take them to your nearest bearing supplier. I would hazard a guess that every big town has one.
When I removed my bearings, front and rear, because I was powder coating my wheels, I wanted to fit new ones as although the removed bearings felt perfect, they might have sustained undetectable damage.
Because they are so cheap, it wasn't worth the risk.
Mine were from a 2020 Ninja 1000 but I would bet they are the same throughout the range.
For what it's worth
Rear bearings 6304 LU ( New bearing package marked 6304-2RSR C3)
Front bearings 6004 DUA2 ( New bearing package marked 6004-2RSR C3)
Both bearings are as common as muck and cheap. I paid just over £20 for the 4 but choose FAG for their quality.
I could have ordered them from the internet for less but bought them over the counter from my local bearing supplier.