Call a couple of the other bridal shops, and some of the major dry cleaners. They should be able to give you the names of some seamstresses/tailors.
Not work on chiffon? How the heck do they think the dress got made in the first place, I wonder? If it was easy, everyone would do it.
good luck! My DIL went thru same thing, lost so much weight before wedding that I literally picked up the dress from our local saint seamstress the evening before the wedding. It was perfect and I'm sure yours will be, too.