Try Custom ear plugs, hearing protection & in ear monitors :: Westone

and the "Find and authorized Westone dispensor or dealer" link on the right side. I've seen these ear molds advertised with Plantronics aviation headsets, so might be a good place to start.

I fly with the LightSpeed Mach 1 and must say I love the custom ear molds. However my molds were done by Sensaphonics and the audiologist who did my molds does a lot of them for musician in ear monitors. It's a weird feeling/process (injecting your ear with an epoxy and having to hold your head sideways for 10 minutes while it cures), but was worth it in the long run.

But, I do know a lot of people who absolutely can't stand the feeling of something in their ear and I think the custom ear mold route is more a personal comfort issue than anything else.