I hammer my vent picks out of coat hangers, after heating them red hot with a propane torch. Hammer them square, and then grip one end in a bench vise, and turn the other( twisting action) with vise grips holding the other end. It makes a nice decorative turned handle. I make a loop at one end, to give leverage, and to provide a place for a tie. The other end is hammered to a long point. Then its cooled in oil, to turn the pick black. I file the pick end small enough and long enough to pass through the vent hole and reach the opposite of the bore. I then file flats at the point on opposite sides to make a small " shovel effect", which helps move the powder aside, and pull out " fines " the may block the vent. I also round the "Point" so I don't stab myself. The pick doesn't have to have a sharp point on it.
In practice, the pick is pushed through the vent into the powder charge, giving a couple of twists back and forth, and then with drawn. Then I prime the pan, close the frizzen, and prepare to fire. The hole I leave in the powder charge gives a place for the heat from the burning prime to reach MULTIPLE granules of powder inside the barrel, where multiple fires will be started to speed ignition.
I have two commercial picks that are made from wire. Check with stores that supply fine piano wire. I simply found some small steel stock at a machine shop and bought a piece from them. It was under 1/16" of an inch, but how much smaller, I do not remember.
Take a spare vent with you when shopping the size, if you don't have a wire gauge. I used drill bits to figure out what the diameter of the vent hole was, and then went looking for wire or rod smaller than that drill size.
It was later that I tried to make my own picks, and now, in have made them for several people, who use them in their wool hats, as tassle keepers, or carry them tied to their bags. The longer pick I make fits men's hands( Shanks are 4-5 inches long) so much better than do the wires, that most men prefer my picks to the commercial wire ones you can buy. The T/C nipple wrench has a wire in the handle/ as does the adjustable powder measure sold by Tedd Cash, and some others. I also have a wire on my Hawken Shop Flint tool, but have never used it. The wires are small enough to use as a Nipple pick, and are really not designed as Vent picks.