One of my favorite green festivals is this weekend at the DC convention center. There are lots of green vendors as well as environmental/green organizations, as well as speakers. You can also get discounts for admission (see their website: if you buy a ticket, you can get 'green festival bucks' and use them as a discount for vendors. Also if you are a student, senior, ride a bike or took public transportation you can get a discount and the GF bucks too), or free admission (for free admission: if you shop at MOM's organic market and bring a $25 receipt from october, OR if you become a member of Green America--which is one of the non profit environmental organizations that sponsors the green festivals--you can get free admission).
Next month (in November) there is also one in San Francisco.
There are others in other cities around the country, but these are the last two of the year.
Check out the green festival website for more information for both the Washington D.C. and San Francisco green festivals
Link to Green America's website