Before you spend your money, try hitting up the Apple store if there's one close by. Just tell the salesperson you lost yours and see if you can "buy" one there. Now, they don't actually sell them separately at the store, but sometimes you can get a genius to just give you one of the spares from the back room. It's not an official Apple policy, but when I was a specialist, I was able to procure a couple for a few customers. Worst they can do is say no, and you buy one online anyway.