Walmart has a freestanding cabinet that fits over a toilet, just $21.98 on clearance (in black).
Perfect to store TP.
The Over the Toilet Space Saver Shelf is an efficient solution for keeping bathroom essentials organized and properly stored. It's made from wood to be strong, sturdy and attractive in any household. This over-the-toilet shelf makes good use of space that is already occupied. It's free standing, which means you don't have to anchor it down and can take it with you if you move without having to disassemble the unit.