1. go to the "search" box on LJ and type in "buffy" That will take you to a page of search results, I believe to entries with "buffy" in them as the default.
2. There will be a blue menu bar above the entries; one of the buttons on it should be something along the lines of "people and communities." Click on it. That will take you to a listing of people and communities that include "buffy" in their tags. (You might also consider searching on variants, for example "btvs" and "Buffy Summers," etc.)
3. Communities will be the first thing to pop up. Browse through until you find communities that look like they might have icons, either communities specifically for icons or general message-board type communities.
4. Browse through said communities until you find something you like.
5. Don't forget to comment with your thanks and which one you're taking.
6. Credit the author on your icon page. In the "Comment" box, type in the LJ username of the icon artist. (For example, for this one the comment says < lj user = "elizalavelle" >, with all spaces removed except the one between lj and user. So when you go to my icon page it has the lj link to elizalavelle