Buy them? I don't know.
Make some? I have a few ideas:
-appropriately-sized tupperware containers, cut to depth
-make some out of fiberglass using the tupperware/margarine tubs as a mold
-cut down a flimsy plastic cereal bowl from the Sally Ann
-cut bottom off quart-size milk or juice jug
-white plastic end caps from a mailing tube (these actually might be best. brilliant.)
All of these lousy ideas come from looking around our untidy family room.
Making things is just where my mind goes when faced with needing a part.