just buy the fabric or strip down a old one, fastex will send you 5 free samples from their website. I chopped down a standard double to a single as I liked three ammo and one utility/grenade pouch on my rig.
easiest kit to alter was the SA assault vest as I had fewer bleeding fingers
I got those bits in the UK, I have'nt had time to start scrounging over here yet.
Can't remember where the 3M place was but the Reflexite was in Oxford the first time, then I think they moved to Oireland when I moved house and asked for more 'samples'........for a road safety project.
Ace supplies whose linky I gave above does most everything you could need in the way of fastners, velcro, webbing and the like.
Pointnorth is an excellent source of small quantities of fabric including cordura and pertex.