Ebay, and that's it. Sega doesn't make them anymore, and made very few when they did.
Unless you get lucky and find one used at an EB or Funco (not unheard of, believe it or not).
If you see something like it at an import .com, and it's cheaper than $100, it's probably the LAN adapter, which doesn't work with any games, but still works with the internet, development stuff, and other, more illicit programs.
And to be honest, it's probably not worth it if you aren't programming anything for the DC. It works with like six whole games.