The main material used for the outside of Zugster bags is 1000 Denier Cordura™ fabric. Cordura™ is a woven nylon fabric with fantastic strength and abrasion resistance.
For waxed canvas bags, I use Waxwear™ brand waxed #10 cotton duck from Herbert Rice Fabrics. This is a tightly woven, 100% cotton canvas fabric which has been treated with wax to resist water. It is significantly heavier than Cordura. When new, it is much stiffer as well, but will gradually wear in, becoming software and obtaining a beautiful patina.
Leather comes from S&H Frank & Company, at 3075 17th Street in San Francisco.