Where to buy a dress that’s not a wedding dress, but not NOT a wedding dress?

Partner and I are gonna get married in front of just our kids and parents by self officiating somewhere pretty in DC. I’d like a dress that’s looks pretty and festive, but isn’t actually a wedding dress. I have no idea where to even start in looking for something like this. Any suggestions?

Editing to ad - I am looking for somewhere I can go in person to shop