You could get them from ebay; I just bought one. Supposedly, this could allow you to play ROMs on the GC. And, since ROMs are legal for games that you already own (this was declared by the Copyright Office
a new exception to the DMCA last month), this is convenient.
Link to an adapter that I bought, ~$20 EDIT: I'm not trying to get banned re the fact that ROMs of cart games that you already own are perfectly legal. If you click the link above and read exception 2, you'll see that, but for the lazy:
2. Computer programs and video games distributed in formats that have become obsolete and that require the original media or hardware as a condition of access, when circumvention is accomplished for the purpose of preservation or archival reproduction of published digital works by a library or archive. A format shall be considered obsolete if the machine or system necessary to render perceptible a work stored in that format is no longer manufactured or is no longer reasonably available in the commercial marketplace.
-of course, this means that once released on the VC, it goes into a shady legal area ("no longer reasonably available").