What is the best way to market a new green wedding planning class?

Awesome idea. Weddings can be so not-green. I’m a photographer. I’d suggest using the same channels that are used to sell services to couples – bridal shows, bakeries, dress & tux shops, limos, banquet halls. You’re bound to reach current and potential wedding planners through those contacts.

What about advertising through organizations of wedding planners? Here is one site. Here is another.

Find out where all the bridal shows are in your area, they tend to be held in local hotels and typical wedding venues, they usually occur around March April time ie just before the summer months (here in the UK anyways!)
Also make sure you are listed on your local yellow pages/white pages web site, you will get some passing trade through them.
Nothing speaks more than word of mouth recommendations, perhaps so some special offers for friends of friends and word will soon spread if you do a good job :0)
GOOD LUCK with everything
xx

Advertising in trade magazines and publications seems like the best way to go. You could also try to buy a contact list from companies that do that – a logical, narrow list would be a great start for advertising your particular service.

Maybe start out by offering your classes under the auspices of local adult education centers. From there, you can get your name out by word-of-mouth. Also, you might want to advertise on bulletin boards at health/natural foods stores.