First you need to download all the plans, labels, templates, etc. Give a huge thank you to all the guys that have worked on this stuff over the years! Everything is here: http://www.hprops.com/labels/http://www.gbprops.com
The main ingredients I used to build my pack are blue insulation foam (available at Home Depot), MDF wood, 1/8" thick pressboard and lots of epoxy. The ALICE pack frames can be purchased from Army surplus stores. Also locate a good electronics surplus store for a lot of the small parts.