Almost anywhere. Grocery stores, home depot, etc. They are by Mr. Clean. They have several varieties. The original is one of the best, but they don't last long. The ones with the blue side last longer, but the blue side is kind of useless. The double strength ones that are wavy aren't worth it, in my opinion.

They come in a blue box with Mr clean on the front. Usually in packs of two or four. In the cleaning supplies aisle. There are knock off brands that work almost as well, too. Mr. Clean are the best, though.

check the link.