Look also at the metal polishes sold used to polish metal pots and pans, silver or even brass fixtures. Toothpastes with tartar control also have a bit more grit than normal pastes, they too can be used to polish plastic.
Sandpaper, you can source a large sheet for a fraction of the price from a local hardware/home improvement store. Just look for those with Wet or Dry Use printed on the back. You should be able to get grits up to 2000 without a problem. Also as suggested check the suppliers that cater to the auto repair shops.
Support your local economy when and where you can, it means jobs for your neighbors.