Sawmill and Woodlot Management magazine has issued a new 2005 Buyer's Guide for Portable Sawmills. It lists most, if not all, manufacturers of portable sawmills available to customers in the United States and Canada. Most of the companies have web addresses.
I also did an interview with Ronald D. Wenrich, sawmill management consultant, on portable sawmills several years back. It's been a very successful feature at About Forestry. Ron gives some great information on the difference between 4 major portable sawmill types - the chainsaw, band, swing and dimension mill. See Portable Sawmills - Interview With a Mill Consultant and Portable Sawmills - What Should You Buy?