iT'S SOMETHING THAT USED TO BE COMMON PRACTICE, FEEDING ONE ANOTHER'S BABIES ,FRIENDS AND FAMILIES.
only a few years ago I was working in children's ward when there was a big shock horror,too do because a "new Australian" had breast fed another baby in the ward.She heard it crying, no one apparently came and she did what she would have done for a friend or relly, she fed the bub.
OMG you have never seen anything like it, the hospital thought they had an assault case coming, the baby was tested for aids and hep b etc.It really was ridiculous and this poor woman who was just helping was made to feel she had done something wrong.
I do know of people who are best friends in the lst few years feeding each others breast fed babies, they both got time out, they didn't have to express and fuss with bottles.both mums and bubs were happy as pigs in mud :)
As to the question, I don't think you would find an ad anywhere for a wet nurse but there are milk banks,esp for premmy's though there was some talk of suspending them I don't know if that has happened I am out of the loop now.