Lots of wire stock will work. Be sure to scrub a coat hanger wire with some steel wool to remove the bronze shellac that they're coated with right down to shiny metal, then oil before inserting in the mold.
Fabric stores that sell knitting materials will often have packages of very small knitting needles, often called sock needles, that are anodized aluminum and come in three or four diameters to a pack. These are trick and cost about $8 a package. Brass rod from a hobby shop or a home center Like Menards or Home Depot is easy to cut to length and works well. Finishing nails in 4 and 6 penny are good in many molds and larger molds will handle larger nails. I've done core shots using a twin injector on 1" Nanos using small brass brads as the pins. There are tons of ways to get the ecore done without breaking the bank, but the clothes hangers work too.