April walks down the aisle in a white dress with baby blue accents in “Fancy Party”

http://www.modcloth.com/shop/dresses/mint-milkshake-dress-in-vanilla

http://www.modcloth.com/shop/dresses/sweet-sophisticate-dress

http://www.modcloth.com/shop/dresses/as-daisies-go-by-dress

http://www.madewell.com/madewell_category/DRESSES/waistdefined/PRDOVR~85617/85617.jsp

I did a quick search and couldn’t find anything that mentioned the exact brand she wore. My guess is that it was Madewell or ModCloth but there’s a possibility that it was custom-made. Quite a few articles of clothing on the show are custom-made. 

A lot of people have asked where you can find April’s jacket from the season 2-4 opening sequence. It’s a Marc by Marc Jacobs hoodie and sadly isn’t available for purchase anymore. Your best bet is ebay or a consignment shop.