Senbon are metal needles with a point at both ends, Typically used by Medical Ninja and can be thrown at enemies dealing 1 base damage which scales with Chakra.
Can be bought at any Ninja Tool Shop for 2 Ryo each, and can be sold for 1 Ryo each.
Also can be obtained from Supply Crates that spawn around villages when users give 100 Ryo to the Summoning Toad in the Leaf Village or Sand Village.