Buying new is often the only choice but before you do, look in your mother's attic.
Vintage and consignment dresses will not only cut down on new material use, but will likely be much more affordable, as well. In fact, the money you save by not buying a new dress, might be able to be used somewhere else in the wedding to be more environmentally sensitive. If you do choose to buy a new dress, buy it or have it made with organic material that hasn't been treated with harsh and dangerous chemicals. Choose bridesmaids dresses that can be reused at later dates. Some styles are conducive to shortening for later, less formal use. Your wedding party will appreciate this consideration, and will probably feel more comfortable in the dress as well. Consider choosing your own dress with reuse in mind as well. If you won't be reusing it later for another event, maybe it can be made with the ability to take it in or let it out for reuse by someone else at a later date.