For the article on cutters who harm themselves see: Self-Harm.
A cutter is an underground surgical practitioner. There are a handful of S&M masters, piercers, and other people who are experienced in doing surgical procedures, and when contacted through the correct channels, are willing to perform castrations as well as other procedures, such as subincision and tongue splitting, that the formal medical community is unable or unwilling to perform. Some have medical training, but most do not.
Cutters almost always work illegally or at least in a gray area, so contacting a cutter is usually done through a series of anonymous and pseudo-anonymous contacts (often former clients who act as a screening service).
How to Find One
Assuming that someone who maintains a public profile doesn't offer the procedure you're looking for, just ask around. Local piercing studios may well know people. Local leather and S&M shops may know people. Try telling your story on mailing lists and describe why you're interested. While most don't post on mailing lists, many practitioners monitor lists to keep up on what's going on and to meet potential clients.
Whatever you do, don't just blindly go to the first person you find. While the first one might be qualified, they might not. Make sure you know all your options, and then make the best choice. Make sure that the person you've picked out to do your procedure is both technically qualified, and that you are comfortable with them on a personal level. If your gut says no, back out.
Finally, if you're looking for a serious procedure, you should know that an ethical practitioner will be just as worried about you as you are about them. As with a doctor, be sure that you can rationally justify your desire for the procedure.