For light-colored treenails, I use the straws from a household corn broom available at most home goods stores. I purchased a hand-held type at a dollar store and got enough straws to last a lifetime. Each one is about 10 inches long and there are various diameters. I pull them through a draw plate to get the exact size I need. A couple of minutes work gives me about 20 or 25 10 inch lengths.
For dark treenails, I use the bristles from a palmyra brush or stable broom. Same deal except the lengths are a lot shorter.
Both types give very strong, uniform treenails.
I dip in diluted white glue then cut them almost flush with little snips. A bit of sanding when dry and it's done.
Terry