We use in-ear monitors in the praise band at church. I've been wearing the $99 Bose earbuds because they sound amazing, especially for a bass player (nice low frequency range). However, they didn't fit well at all. The little earpieces didn't sit in my ears well and they frequently fell out. I was constantly having to reinsert them. So I decided I wanted to to convert them into custom fitted ear buds, like the kind the pros wear. However, I didn't want to go to an audiologist to have custom molds of my ear specially made, nor was I brave enough to use a caulking gun in my ears. So while Googling how to do this yourself, I found the website for EarPlugsThatFit.com. Which also advertises the same product specifically for custom fitting earbuds. I ordered a kit, which arrived yesterday and promptly turned my uncomfortable earbuds in to awesome ones. They send you 2 canisters of different materials, which after you combine them and insert them into your ears will cure into custom fitted silicone earplugs. For only $16.49, this was WAY cheaper than a custom molding at the audiologist. I've been wearing them for hours tonight while working on the computer and I almost forgot I had them in they're so comfortable. I highly recommend this product to anyone who wants a custom fit for their earbuds.
UPDATE (April 21, 2009) - As you can see from the comments to this post, Earplugs that Fit has sold the company to Radians.com , so if you're interested in this product, you can find it there.