If you have a Hot Topic store near you - they currently have a TON of cool HP stuff out for the new movie. I just got myself some cool new t-shirts and a set of earrings that had owl earrings, lightning bolt earrings and earrings of Harry's glasses - very cool stuff.
They also sold a journals with the Deathly Hallows symbol on the cover which could be nice if she likes to write.
Outside of Hot Topic, there are some cool books I've stumbled across over the years like "The Philosophy of Harry Potter" (this is part of a series that does"the philosophy of..." lots of different pop culture books, movies and TV shows), and there was a book I read this summer that was all about the writing of the Harry Potter books, if your girlfriend is a literature geek (I'll havetocome back here and post the title after I post since the book is on this iPad!).
And then there is one more thing... You can a little bit of a personal touch with some JK Rowling flare. Yousaid you guys have been dating 7 months. 7 is a pretty significant number - even in Harry's world of magic. writing a letter or card to your girlfriend explaining how important she is to you and connecting it to the significance of the 7 may be a very nice touch!
...I'll be back momentarily with more "stuff"
BOOK: Harry Potter's Bookshelf: The Great Books Behind the Hogwartz Adventures by John Granger
REFERENCE FOR "SEVEN" STUFF ( this is not an exaggeration - JK put these references in purposely):
http://harrypotter.wikia.com/wiki/Seven
All the Best! I hope your girlfriend has a very happy birthday and please remember, it is never about how much you give, but HOW you give. Just think it through, don't stress it too much and give with joy!
