You can also buy them on ebay. Many auctions are for lots of them, but there are some that are just auctioning off 5 or 6 of them.
And yes. Certain ones you can replace the battery in. It will state it in the auction. Otherwise I think that they last for about 24 hours of use. (you can turn them on and off so it's not like they are good for only a day, they could last you quite a while if you use them correctly.)