Affiliate Marketing Secrets

In Tim Ferriss’s prescription for creating a business in the 4 Hour Workweek, he advises finding a market first, then making a product for them.
I have followed the steps in the book and identified a few niches I could market to. For each niche I have found several small trade magazines. So far, so good.
The problem is, after contacting the ad directors at a few of these magazines (to ask for back issues and their current rate card), they all hit me with a barrage of questions like “what’s your product URL?” and “what is your product?”
This is a catch-22, because I want to validate the magazine before I come up with a product idea, let alone launch a website.
What do I tell these folks? Has anyone here created a muse successfully following Ferriss’s steps in the 4 Hour Workweek? What’s the best way to ask for this information when you DON’T EVEN HAVE A PRODUCT ?
Thanks in advance!