I just did a search on Etsy for them and there's quite a few on there. I think they're cute.
I'm going to make some in the school colors around here. I think they'll go over well. I am thinking that I could slip the ponytail holder thru the yarn before I start making the yarn ball--and by the time I go to cut the strings the ponytail holder would already be towards the middle of the ball and would be secure. Would that work? I'm going to make some Halloween colors as well. OOhh, another project... heehee