Find a female friend that you can trust and that mostly trusts you. Then just one day compliment her hair as you're hanging out (it's better if you're in a private setting rather than out in public) and ask if you can play with it. It will feel weird to do so, but if she trusts you, then she'll probably say yes.

I myself have a best friend that is married, but she lets me play with her hair all the time because she trusts me and knows I'm not trying to get in her pants from it.