These are what Kathy recommends for holding the booklets and folded music we use in our black binders. They're almost never found in stores, so you'll have better luck ordering them through the web. Here's one aggregator source, which has different quantities and prices.
And if you don't already have them, plastic sheet protectors are also a must. Those you can find at OfficeMax (and say hi to JP!), Staples, Wal-Mart, Costco, Sam's, etc., or you can get them through the web also.