There was a program on channel 4 about this, you may be able to watch it online? I think it was called 'Other People's Breastmilk', but I'm not sure.
It's not something I would ever feel comfortable with for me and my child, but the attitude that breast feeing someone else's child is wrong is very much a modern and western train of thought. In some communities children are very much raised by a family, or community of women. In the past in england, people employed wet nurses to care for their children.