I'm a 28yr old Sunni man. I am wondering if I can do istikhara for marrying (or not marrying) a girl from a particular country.
You can do istikhara on anything, any matter you are going to do. And specially when you are going to marry, it's a big step and you should do istikhara.
I can't think of any reason you might not do istikhara on anything you want to do.