Somewhere like Home Depot or Lowes will have them.

An alternative is to use ready-meal plastic food containers from something like Nissan noodles (there's a variety that has a plastic container versus paper), or Hormel "Compleats" meals. Just eat the food - I keep a few as a quick cheap lunch at work if I forgot to brown-bag it that morning - run the container through the dishwasher, and cut an access hole with a pair of scissors.