G.M.
There is a great resale shop in Strongsville called the Neverending Child. It is on Pearl Rd in the Staples Plaza. They have a whole rack dedicated to formal wear.
If you go further down Pearl Rd by Walmart they have the Salvation Army Thrift store also worth a try.
Also - store's like Target and TJ Maxx are starting to put out their Easter clothing already - that could work depending on how formal she wants.
Hope that helps.