My brother and sil asked dd who was 6 at the time to be the flower girl for their wedding. Long story, but my brother and sil's 3 year old son was the ring bearer. Anyway, they wanted a formal wedding, etc. I got the feeling that sil wanted dd to go to a bridal shop and get her gown there.
I worked in the Bridal Dept at Leggett's many years ago, so I knew we were looking at $150-200, easily, especially with a crinoline, headpiece, etc. So I asked sil to let me know where she was getting her gown so I could look at it. They lived in Florida, and we live in Virginia. I went to David's Bridal to see sil's gown. Having worked in Bridal, I knew I could find a dead ringer for her gown for less. Sure enough, Dillard's had first communion gowns for $70, and I would defy anyone to show me the difference.
OP, does SIL live in your town? How much time do you have? I'd look at her gown, and see if you couldn't find a match for it somewhere. Unless she lives in the same town, she'll have no idea where you bought the gown. Look on Ebay too. Good luck!