Hi!
Not sure about the size, but I doubt if you can buy it separately. I know that if you buy a new shell (for example at AliExpress), you will get a new square nut, which you can transfer to your current back cover.
Example: https://godofgamingshop.com/collections/game-boy-advance-sp-parts/products/game-boy-advance-sp-shell
You could also contact them if they have a square nut available.