It argues that newspaper adverts are the most common method used by men to access sexual services.
The WI is encouraging its members to pass on details to the organisation, although it has yet to decide what it will do with the information.
Spokeswoman Ira Arundell said the WI had the option to "name and shame" newspapers which published such adverts although its "absolute focus" was on helping the women concerned.
"We want to raise awareness and spread the message about what is happening with these girls," she said.
Ms Harman has already won the support of The Newspaper Society, which represents local papers, to discourage such adverts while several publishers have agreed to stop publishing them.
Members of the Women's Institute talk about their concerns
In her speech to the WI, she said the sex trade was "the modern-day version of the slave trade" and urged members to look through local papers to see if they carried the advertisements.
"I hope that members of the National Federation of the Women's Institute will help protect women from being brought here, from abroad, and being forced into the sex trade," she said.
The initiative comes as a survey of 143 WI members found nearly half of them had suffered violent or sexual attacks or knew someone who had.
The research was part of a Bristol University project on violence against women in rural areas.
Ms Harman said cases of domestic violence incidents had halved in ten years. She said since 1997, there had been a 58% fall and that 73% of domestic violence cases bought to court now resulted in a conviction.
But she said that while "real progress" had been made against domestic violence, efforts should be re-doubled.
"But, here again, we still have to continually challenge reactionary attitudes, that domestic violence is a private matter, it's a private matter between husband and wife, that it's not for the public authorities to intervene or that - and this is the really pernicious one, she must have bought it on herself.
"So, there's still a mountain to climb to end violence in the home."
The WI members who were surveyed criticised support services for victims of domestic violence, with 90% feeling the police did not give the support they needed.
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