I've never ventured into this aspect of photography, but what Jerry Cunningham said makes a lot of sense to me.
Many years ago I worked with a couple of sluts (guys) who had the opposite approach and claimed to be highly successful.
One of them would go to the Santa Barbara area beaches and ask young ladies if they would take their tops off so he could take a picture. He claims that about 1 in 8 would go along with it. He had pictures to prove that he was occasionally successfull with that approach.
The other guy thought nothing of rolling down the car window when stopped at a red light and asking a women in the adjacent car "How'd you like to (insert vulgar term for intimate carnal experience)". He claims to have been successful in 1 out of 4 attempts. He seemed to have a perpetual smile on his face, and he was frequently late coming back to work from lunch, but I tend to question his "success" rate.
So... with a polite and legitimate request, I don't see why models would be too hard to find. Me, I'm still to shy to ask!