I love Nordstroms online dress department. I'd browse the cocktail party options. They have lots of reasonably priced options. I have had great luck with dresses from here. nordstrom.com/browse/women/...
I just had an outdoor mountain wedding that was "cocktail casual" and ran the risk of getting chilly at night, similar to a wharf (maybe?) Anywho, I got a light pant jumpsuit and dressed it up a bit with a chunky necklace and updo! From Marshall's $20, including my picture because I was back and forth on outfits for like a month and proud of my final choice.
I second Ross and Marshall’s! Any fun colored summer/floral dress or romper would be perfect! My dresses & rompers I get the most compliments on are always from Ross or Marshall’s. You can’t beat their prices, so I just keep going back!