Free to join trade associations

Does anyone know of a directory of free to join trade associations? Needless to say I looked but couldn't find. It struck me that it would be just the thing to make my publshing related white hat site look more credible without filling it up with testimonials. (or maybe this is a grey hat tactic, I'm not sure).

I spent quite a long time looking searching Google. And none of the results in your search are actually free to join trade associations.

But thanks for taking the time to look for me.