Where can i find an Informal Garden Party type Wedding Dress?

I am marrying my 27+ year partner on the 28th anniversary of our meeting. We have both been married before and are in our late 70's, but are still actively engaged with life, curious, and in good physical shape. We are getting married at a nature center with an informal garden/farm vibe and sunflowers as our main theme (we're getting married on the Summer Solstice.) We are inviting about 120 guests, with half being extended family. I was a hippie wanna be in the 60's and this feels right to both of us. I am envisioning a sundress that is either at knee length or halfway between maxi and knee length that is a cotton type floral with a light white, off-white, or very pale blue background. Obviously, wedding dress venders are not showing this kind of dress so does anyone have ideas as for any readymade designer or brands? I'm not looking to spend more than $200 - just a really attractive summer frock to match the informality of this amazing event. The lunch will be buffet rather than plated. My fiance is wearing (very nice and new) jeans and we've discouraged coats and ties. We'll have country music with a fiddle, banjo, guitar and singer and maybe some circle dancing to include folks our age, our kids' ages (40's and 50's) and a ton of grandchildren (well, at least eight.) Looking forward to some great ideas from this terrific forum.