Where do you buy a whiteboard?
You can make your own.  Otherwise, questions about where to buy stuff are not in scope for this site.

If one of the walls can be repainted and is otherwise suitable as a whiteboard, just paint it over with this.



More likely, as a renter, you can't modify the unit or paint it, so you'll have to buy a sheet of fiberboard, plywood, etc., hang it, and then paint it.