I don't remember where I got it.
There are similar fabrics in Revolution in Fashion 1715-1815, p.38 has the closest. Here is a detail of the fabric in the book. (If you haven't got this book, you need to see it! It's terrific!)
Pretty similar, hmmm? Not perfect, but it's next to impossible to get a dead ringer in modern fabric.
My gown is almost a gown in shape, but needs its sleeves sewn on, front skirts, robings and a ton of trimming!