If you know a Dentist (or someone in this kind of field), they can do it for you.
Actually, I talked to my Dentist, who happens to be my Dad, about custom fit earplugs. He said he could make some, and brought two kind of pastes that had to be mixed. After 2-3 trials to get the right material, he just put some of the mix in my ear, custom fit it, and add a neck rope. Voila ! Free, really custom fit, most comfortable,ear plugs.