buy a black keyboard of your choice.
buy white spraypaint and clearcoat.
disassemble the keyboard.
paint the outer casing white. ideally multiple light coats.
clearcoat. ideally two coats.
under $10 provided you already have a keyboard.
or i suppose you could find a keyboard manufacturer who offfers white and black variants and just swap cases. this might be hard as many manufacturers offer black but not white options and more expensive as you now have a black keyboard with white keys and now a white keyboard with black keys instead of just one keyboard.