Or you can go to any hardware store and find some "1/4 inch double braid poly rope" in a color you like and follow this link to instructions on how to make your own. Rope Halter with Fiador Knot
There are two different sets of instructions for two different ways to tie a rope halter. I like the first one.
It can be complicated and confusing the first time or two, but it's not too terribly difficult to figure out with a bit of practice...especially if you have ever done macrame.
Edit to add: Most of the 1/4" rope bundles are 100 feet and cost about $15. You only use about 20 feet of rope for each halter, so you can make 5 halters from one bundle. About $3 each!!