Here's a quick tip, check your local thrift stores year round and pick up any binders in good condition. Shopping at your local office supply store gets real pricey when you need to buy multiple 2-3" binders.
When shopping in your local second hand/thrift store make sure they are in good shape and will last. They can't be ripping at the sides and they must close well. My kids prefer that they have a clear front pocket to add their own cover to make it personalized.
Since there were so many cute designs out there, I thought I would share a few that you can print out and use from sites all over their web. There are so many more. This is just a sampling. Enjoy!