ammo you never pay for
You do not craft or buy these. Carry a blowpipe or a blowgun anywhere in your inventory and cutting grass or flowers starts dropping them, with flowers paying out several at a time. Something else cutting the grass counts too, so a falling star landing in a field leaves a small pile behind for you.
what that means early on
It makes the blowpipe the one starting weapon with no supply problem. Sweep a field of grass on the way out of spawn and you are stocked for the first few nights without touching the Merchant. Later on the damage stops keeping up, but by then you are cutting grass for herbs anyway and the ammo piles up regardless.