The RB hides are great but with shipping will be a bit pricey if you just need to order one. Usually I throw a few on whenever I need to order something else.

Otherwise anything sized correctly, dishwasher-safe for easy cleaning, opaque, and fairly thin so you can cut an access hole into will work. I've seen plastic flower pots, some of the Hormel or Ramen meals that have pastic containers, plastic bowls, tortilla containers, etc. used.