When I first started punting years ago and used agencies exclusively it was no problem to find a girl on a Sunday. You can just ring one number - the agency of your choice and they'll find a girl who's working. Now that I just rely on AdultWork for punts and call the numbers directly, I agree it can be very difficult to find one free on a Sunday. The last time I tried, I travelled to London, spent an hour or so phoning all different girls and then just gave up and went home!