I got mine at Target on clearance from the Back to School section. Yes, they might look a little juvenile but I like them! I have also seen some at Dollar Tree (they were solid colors). So its a totally cheap way to do a little organizing!!
I use the one on the left as storage for the magnets I'm not using at the moment. The one on the right is for the photo invitations and birth announcements my friends send me...I save them all!