Hi L.,
When I was a kid, I had a few pen-pals who lived overseas that I got from a kids' magazine I subscribed to. There was a section for choosing and offering oneself for pen-paldom (good word, eh?). I had some long term relationships and got a lot out of doing that. I'm sure today, the magazines for kids are more about how to kiss a boy (or worse!) than about that!
My daughter, now 11, wanted a pen pal a few years ago and we joined some online kids pen-pal (safe) site ($7 per year or something), that was supposed to help the kids find a pen-pal overseas, but it ended up that they just chatted on this site and nothing in depth, so it was not at all what we, (I), was looking for. Then, on the only other site I frequent and post on, another mother asked me if she thought my daughter, (we would all write in about our kids in various contexts), would like to be a pen-pal with her daughter. At first we had a rule that it could only be via pen and paper letters in the mail, but now, since they have established a nice friendship and like some of the same sites on line, we allow them to do both. They love to get each others hand written letters! I am sure this girl is my daughter's very best friend and they have never met!
As for getting one overseas, if you sponsor a child, as mentioned, you can specify that you are looking for one about the same age and gender. They send you a photo and info and let you choose if that child seems a good match before you get started. It usually costs about $30 per month and is very rewarding.
Otherwise, ask around people whom you know are from, or have contacts in other countries. Someone may know someone who has a little girl your daughter's age who would also love this opportunity. Perhaps you can contact a school overseas or have your school contact your "sister city's school).
Good luck. It's a great experience for kids! I'm so pleased to hear kids who are still interested in this kind of thing!