I got mine at Home Depot, but I've heard HObby Lobby has a big selection, too. Take your needle gauge thingy along and find a dowel that fits size 13. A LITTLE too big is okay--you can sand them down.
Here's a tip: When my son was helping me make mine, after cutting the dowel into equal pieces, he put one end in the drill (as if it was a drill bit) and turned the drill onto a grinder. I did the same thing against sandpaper to make it smooth. It went pretty fast this way. It works better on one end of the needle than the other, but that way one side is SUPER easy to do. If you don't have the equipment to do this, a pencil sharpener works, too, but makes the tips a bit longer. HAVE FUN WITH THIS!