you're probably best of making them yourself.. or use a converter like awave studio to convert soundfonts for example to xi format. You might lose some quality and such (I don't know the specs of the xi format, but I don't think they're that good since it's a pretty old format nowadays).
only site with downloadable xi's (as far as i know):
http://www.kiarchive.ru/pub/misc/sounds/samples/ft2/you can buy the no.1 sampling cd
here