How do you get other lockers?
I see in the locker room other lockers.
But looking around I havent seen anywhere where I can get another locker type.
So where do you get them?
The bigger locker will be available after you've hit level 15.
When you've reached to level 15, the locker room gets exclamation mark (!) to let player notice that the player's locker has gotten bigger. After that point, the Hotlantis (general store) also starts selling some new items, including colored lockers.
Above is based on my experience, but I've also found https://www.polygon.com/guides/23349035/splatoon-3-locker-upgrade-big-pink-red-black-colors-unlock to prove the information above.