I'll be happy to help.... but I must get this off my chest immediately:
If your girlfriend is such a super-duper Bettie fan, why isn't she posting here?

And if she is such a passionate Bettie fan, isn't she at least reading this Forum? (How else did you find out about it? Or are you a Bettie fan, too?) And if she is reading this Forum, won't this thread give away any Bettie birthday surprise from you?
Narrowing down what a Bettie fan enjoys is impossible. Bettie's fans' interests are as diverse as the fan base. You'll discover every conceivable age range, libidinal attractions, types of collections, spiritual connections (or lack of them), tastes, etc., etc. You get the picture. The long and short of it is that what one Bettie fan would enjoy may not appeal to another.
It would help to know how old your girlfriend is, and what about Bettie attracts her. What sort of Bettie collectibles does she already have? Does she like to collect Bettie's photos, books, figurines, or is she more into Bettie merchandise that she can wear? If so, what about something with a bit more "romance" -- like a "Love, Bettie heart charm" that she can wear around her neck and close to her heart, so she always thinks of you and Bettie?!?
If you'd like to check out a vendor of officially licensed Bettie products, with three pages of Bettie items, you might want to surf to Retro-a-Go-Go. Perhaps your girlfriend would enjoy a Bettie Page parasol. You can see one in the animated gif on the homepage. They sold out like hotcakes when Retro-a-Go-Go attended a show in Vegas and they're on the list of items to restock. If a parasol won't cut the mustard, check out other options:
http://www.retroagogo.com/servlet/StoreFrontThe owner is a Bettie fan and I'm sure she'll help you pick out just the right item to suit your girlfriend's tastes and your budget.
Patti
P.S. Looking forward to posts from your girlfriend -- and if you're a Bettie fan, too, hope you'll join in more often! Welcome to the Forum.