Sustainable Dresses

I’m going to a wedding in a few months and have just gotten into sustainable fashion/living. Being fairly new in trying to slowly replace pieces in my closet with organic materials and sustainable brands. I’m going to a wedding in several months and have time to look around but I was wondering if anyone knows of a new or used dress similar to this Reformation one. At first I thought I’d be okay since it’s 100% silk however I’ve been reading that they are a fast fashion brand and I don’t want to support that. Thank you. Any other recommendations for all clothing would be helpful as well.