http://www.mini-itx.com/ U.K. suppliers of everything you need to build a mini-itx computer. They even sell pre-bult systems, and have a large library of articles about mini-itx system in "unique" cases. Interesting reading.
http://www.exoticpc.com/index.php One of the most impressive looking "quiet" suppliers - these really know how to make a website. Unfortuately some of the claims are a little unbelievable (14dBA for a PSU, which is obviously not any standard measurement), but none the less they are well worth having a look at.