I purchased an old fleece blanket from a thrift store (that supports a local animal shelter, naturally) for around $2. From one blanket, I could get eight approximately 10"x10" squares. I was going to hem up the edges, but who cares!? Not my dirty floor.
I actually think these work better than the actual Swiffer pads. They glide along my floor a lot nicer and pick up the smaller sand particles better. When I'm all done, the large clump of dust and dog hair peels right off to be thrown away. The rest goes into the washing machine and comes out perfectly clean.