Yeah I'd say a #8 is plenty for this app; good lord I don't know what I'd need bigger than a #10 for! I think I bought my last zips from RW for doing zip mods to assault packs for Crye PC's. So yeah they have #10, GI.
Sewing tutorials are few and far between. I used Poynter's Parachute Manual, Vol 1 for many years. It has common sewing practices for parachute riggers, which are a perfect match to what we're doing, since it's the same materials. It may no longer be available, since Sandy Reid came out with his, which has replaced Dan's manual as the official FAA riggers guide. It's called the "Parachute Rigger Handbook" and is available at Para Gear's website, although those gonoff's want 85 bucks for it. It used to be online so maybe look around for it.
There are also a few books on outdoor gear sewing which may be helpful. Annie Getchell did a nice one.
Sometimes your local sewing stores will offer classes on making stuff. So if you can suffer through one of those, you will learn basic practices for clothing fabrication. Same same for the different "how-to" books out there.
I find sewing is sewing, so if you can talk to anyone, whether the make prom dresses, upholstery, boat canvas, quilts, whatever, there are useful techniques you can pick up from any community. Hell I even made inter-bulkhead insulation blankets, and inter-stage blast shields for Atlas, Atlas-Centaur, and Titan-Centaur space launch vehicles. The technique's I learned as a rigger were directly applicable to aerospace manufacturing.