Come in 1-4 inch lengths with a loop on the end. Usually come in 24-20 Gauge wire. You may find it more economical and less waistful to use a continuous length of wire and make your own loops.
Also come in 1-4 inch lengths and 24-20 Guage wire. They stop the beads from slipping off the end. There are a variety of designs, e.g. ball ends and other fancy ends. You can alternatively make your own headpins.