Do you have a Target nearby? They have an awesome "Dollar Spot" usually as you walk in the front door. I bought my DS a bunch of wooden cars, trucks, emergency vehicles, etc. there, as well as some cool pencils, stickers, stampers and ink, and on and on.
Or, how about getting something all the kids could enjoy together (a subscription to NetFlix or Blockbuster online?) and then smaller individual stuff for each one?