I have a wedding to attend Labor Day Weekend in Sonoma County, CA.
My arms and shoulders need to be covered and my dress a modest length for all events due to the religious beliefs of the bride and groom and their immediate families. It will be an outdoor ceremony with an indoor party afterwards. My collection of formal/dressy occasion items are too snug right now and I cannot count on them fitting much better in a few months.

Is it too soon to start shopping for a dress? Should I wait? Any thoughts on where I might look for a nice dress for such an occasion online?

Whatever I choose needs to be transported across the country in a carryon size suitcase. I do not have a separate hanging garment bag for travel.

What materials travel well and would be appropriate for the weather that time of year? I may not have access to a steamer or an iron.