So, I decided to make some of my own...
After doing a little Pinterest / Google research I found that fold over elastic sells for about $1 a yard (which will make roughly 3-4 hair ties), and you can buy a variety of colors, patterns and textures. My favorite place to purchase them from so far is Etsy, but you can buy them from Amazon and lots of other online retailers.
To start I purchased 10 or so yards in various colors and patterns. I wish I had bought some in metallic gold and silver too -- next time! Honestly, the hardest party was picking from all the variety.
For this project you'll need:
5/8 inch fold over elastic
a standard piece of printer paper, for measurement
To make a pony tail: use the length (the longer side) of the printer paper to measure out your elastic. Don't pull the elastic tight or stretch it when measuring, you want it to just lay normally across the paper.
To make a headband: follow the same steps, but measure the elastic to 18 inches using a ruler or tape measure.
You'll want each piece of elastic you cut to be about 11 inches long (the length of a standard piece of paper). I cut mine a little shorter since I have fine hair. If you have really thick hair you'll want yours to be a tad longer.
Simply fold the elastic in half to the ends measure up evenly, and tie them together in a tight knot.
I've heard some people say to singe the ends with a lighter to keep them from fraying, and you can totally do that too if you're concerned they'll fray.
I think they make a nice little gift too to accompany any gift. Stretch them onto a piece of cardboard for nice presentation. They're an easy, inexpensive and FUN addition! The headbands are great for wearing to the gym too.