I just found these canvas-type baskets this week at Wal-Mart. They were in the closet organizing area of the store. They measure 9" x 7.5" and are about 6.5" tall.
The best part is they were only $1.97! I thought this was a great price!
I have tons of zip-lock bags with "flashcards" that' I've made. One basket will be for fluency cards, one for picture cards for writing, and the last basket will be for flashcards to help students in grades 3-5 with state assessment indicators (prefix/suffix, fact and opinion, and author's purpose).
Something I've done to help keep my flashcard sets organized is that I always copy them on different colors of paper. That way, if one card doesn't make it back to where it belongs, it is very easy to figure out where it goes!