I don't know if my comment is worth jack as I'm also a newbie at these topics. But I've found that swimming through applications notes, videos and forums to be a great way to find out more about placement rules, general design, etc.
Where I'm from in Latin America, those kinds of jobs are also rare/scarce and I have no mentors, so I kind of get you. I would also think on maybe picking a project on your own that's complex / hard to do without proper understanding of layout. Maybe much higher speed circuits? Long communication stuff, radio, etc. You could then build your portfolio with that(?)
I've been going through a book on electromagnetic compatibility which is kind of cool but really hard. The one I'm using also talks about PCB layout. You could try that route if you're a self-learner.
That said, to answer your question. I would also like to know if affordable / good certs exist for that kind of thing. This one
http://www.sigcon.com/Pubs/HSDDsem.html is a course from a Dr. who wrote a book on high speed digital design.
But it costs $10,000 for the whole thing. And I also have no idea if it would be worth it. Probably not..? Chances are you and I can't afford that. I don't know if maybe they'd give discounts?