There are two methods of getting custom minis:
- modification of existing miniatures
- having custom sculpts made
Modification is the easiest, and in skilled hands, produces exquisite minis. I'm a not an expert at minis mods, but even I can make halfway decent mods by careful use of a jeweler's vice, jeweler's saw, and superglue.
Custom sculpts are generally not going to remain custom; most of the sculptors I've encountered will take a commission at reasonable rates only with the proviso that they own the resulting sculpture and may sell it commercially. Otherwise, the prices get high quickly.
I know one custom minis maker: Forge of Ice. I don't have a direct contact for him, but the link is to his profile on a board I know he frequents.
He lists http://forum54.oli.us/index.php?topic=5205.0 as his "homepage"...