You probably won't find any local stores carrying what you want. Local stores only sell frilly crapola for dresses and quilts and stuff.

You only need the very back of the bag to be breathable, not the rest. People are always trying to make bags out of goretex and stuff, and you really don't need breathable for the whole bag. A waterproof liner would be fine, as long as you don't have solid cordura up against your back.

I recommend getting some samples from companies that sell sunforger canvas. I can't recommend any offhand, but for other outdoorsy fabrics I can recommend http://www.seattlefabrics.com They sell all sorts of cordura, nylon, and other industrial strength stuff for people making their own bags.

For buckles, go to http://www.itwnexus.com and place a sample order. They'll let you order up to 15 items for free as samples. They won't necessarily send you exactly what you order, but my sample order got me enough buckles for a complete messenger bag, including compression straps.

peace,
sam