This site looks to be chock-full of places to retrieve Mac thorns. Odd that my Mac fonts include the eth (which doesn't reproduce here) but not the thorn. You gotta be careful to choose one that'll reproduce both in word-processing and on websites. I'm still digging through it all. The best bet seems to be to download Icelandic character code.

http://www.georgetown.edu/cball/oe/oe-fonts.html