I have made countless possible bags (different from muzzleloading shooting bags) for re-enactors and for those who don't care for modern fabrics. These are super easy to build and I highly recommend making your own for those 'personal touches' if you have the time. The materials needed are very minimal.
Shortly before an injury took me out of the leather shop for a spell, I had found an old piece of brain-tanned moose hide. Wanting to make myself something...for a change, I decided to use this leather for a medium-sized possible bag. I wanted nothing fancy - just fairly plain and something that would sort of blend into the woods like a commoner.
Before I started making the bag below, I soaked the brain-tanned moose hide in walnut dye (made from walnut hulls and an ancient recipe), for 2-weeks, to give it a more earthy shade. I completed the bag with a hand-woven strap (strap is linen warp and wool weft for decades of use), hand-forged iron buckle for the strap adjustment, and a handmade copper button to secure the flap when needed. A plain old historical bag, like this one, only takes me about 3 - 4 hours to complete (other than the dye or any tooling required). The sheath I made in the photo has a flap on the front (popular before there were belt loops). I often wear this sheath tucked in behind a sash (like Ron Hood wears). The sheath flap hooks over the sash to keep it in place and is worn in the front (works wonderfully).
I have a couple of reasonably priced possible bags made by Pepe's that I like. These are quality bags and will run you just a few more dollars than the store bought variety but worth the extra coin.
http://www.pepespossibles.com/
Crazy Crow
http://www.crazycrow.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=800-100-000 has some inexpensive bags but you will have to add a few of your own touches to give them some personality (if that is important to you).
Some of my friends on
http://www.historicaltrekking.com/ build quality possible bags and you can wander over there for some excellent advice and sources.
My piece of old, thick, durable, butter-soft, brain-tanned moose.