There's only one place to start; with your girlfriend. If you ever expect your relationship to endure this sort of lifestyle you're going to need to communicate on a level far above seeking advice from strangers on the internet.
By asking this sort of question here you'll get two main categories of responses. First, absurd answers from folks who are tire-kickers. Things like "Have a BBC meet you at a bar and hit on her." Ignore all that. Second, you'll get real folks who tell you to communicate your desire with your girlfriend. That's the only way go to about it.
Is her desire just a fantasy or does she actually intend on following through with it? Women don't do this stuff because their boyfriends want them to. They do it when they're ready to do it. Be supportive, let her be herself, but don't be demanding. Remember, it should be about seeking pleasure for you both as a couple, not fulfilling your won sexual motives.
