Go on Adultwork, search for escorts and tick the "Being Filmed" box. Then politely ask if they are happy to take photos as part of a booking.
Be prepared for some to say "yes, with my camera, no faces, then you buy access to my private gallery to see the pictures". Others will be more open to it, I've had numerous bookings over the years where photos are included in the price and faces are shown.
It does depend on what you want to do with the pictures though. I've only ever said for my own private collection. Means that if they ever see the light of day beyond me, there is the potential for serious reprocussions. So make it clear if you want to use them, sell them etc as it will mean getting the model to sign a release form.
There's plenty of resources if you want to take it more seriously. Try
External Link/Members OnlyThe site is a bit dated in appearance, but it's a useful resource.