High end resale shops might have a few vintage designer gowns, but they will want to sell them ... not rent them.
The only place that I can think of, that might have any selection at all and is willing to rent them out, is at some the costume houses in Los Angeles. Western Costume in Hollywood is one. Universal Studios has one behind it's backlot, if they still do that. There is one in North Hollywood/Sun Valley area, but I forget it's name. There are more .... I just don't have the information handy. I found them by Googling. I know that they are not in your area, but it might be worth a trip down here.
You will have call ahead and find out how to set up an account with them or how you need to cover possible loss and or damage and about how much time you for returns and if you can ship them back ... but it's not as hard to do as it sounds. Then you can shop in their warehouses ... they have stacks of racks, up to the ceilings, miles and miles of wardrobe. They have huge ladders that roll, so you can get up there. Bring an assistant to help lift things down to the rolling rack they will lend you. Each type of clothing will be in it's own section, so vintage gowns will probably be in one area of the warehouse. Of course, you will need to know what's real and what isn't, if that is important to you to be authentic.
