Does Apple provide replacement tips for their In-Ear Headphones? One of my earbud tips has been lost. I'd like to replace it. I'd prefer not to use a third-party service, as the quality of the material is never as high as Apple's.

Do you know if Apple will replace them?
I know it used to be not a big hassle to change them. You could just send them to Apple and a few days later, the new earbuds arrived. But since many people took advantage of this system, Apple came up with a limit (both in quantity as in warranty time). 

Now days, my best guess is to go to the Apple Store and ask if they will replace it.  

But why would you exclude non-Apple earbuds? There are some very good (and cheap) ones. For example on DealExtreme or even on eBay.