However the book says nothing about where these portals might appear.
I'm guessing that that is deliberate. It can really break your game when a player flips open his stack of books and points out a portal exactly where they're trying to go. These things show up at the speed of plot, and that's a good thing.
However, if you do want to make it arbitrary, here's how I'd do it. Any location major enough to have a background feat listed in the FRPG has a known portal in its capital. Access to these portals may vary from place to place, but they're always there.