You could use metallic embroidery floss/thread found at any hobby store for less than $2. Good luck!
This; I don't know if you have a Hancock Fabrics or a Jo-Ann Fabrics in California, but they both sell Sulky Metallic embroidery thread that is a perfect pony tinsel match. If you don't have one of those stores, call up your local Fabric stores and ask them if they carry the Sulky metallic embroidery thread. You could always buy online from one of those stores as well.
It comes on a spool, here is what it looks like when cut!
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loginGood luck! If you don't have access to any locally and buying online is a hassle, I can help you out if you need.