Besides the usual outdoor sources, I buy from two other places that may not be generally known about in this community. The first is lowysusa.com, and the second is diytactical.com. Although they cater to more military or tactical-oriented gear, the earth tone colors of browns and greens would work just fine for civilian applications as well. Lowys has minimums, DIY will sell most stuff by the yard or piece.
Another source of light-weight materials is para-gear.com. Although they can be a little pricey.
I have also found that by going to trade shows and establishing relationships with a few of the mfg's, they will usually be willing to help out home-sewers with some odds and ends.
On needle sizing and such, I usually sew with a #16 or #18 needle for light weight fabrics, with no. 69 nylon bonded thread, and 6-8 SPI. This is brought forward from parachute work, and was the set up for canopy repairs. Since it is similar to the light weight work you guys are doing, I would think it would work quite nicely as a starting point.