I want to fulfill his fantasy but I don't know how to get the girl.

Some fantasy is better left in dreams, Why do you want to do this? What type of sexual acts are you willing to do with the girl ( if any) Are you going to sit there and watch him have sex with another girl ?

It's his fantasy. I suspect he wants to pick the other female.

You are okay with this?

How long have you been attracted to other women sexually?

I'm betting you are NOT sexually attracted to other women if you're honest with yourself.

Tell your husband that this one stays in fantasies.

Is this something you really want to do? Does it make you uncomfortable? If you want to be with another woman sexually just to please him than I would be open and honest about how you really feel. Don't ever do anything that compromises you emotional and sexual health and well-being.