Hopefully your partner isn’t as well-versed in muslins as Mr. Tilney, because most modern gowns are not made of non-synthetic materials. Luckily, floor-length gowns with high waists are very in. The sleeves and decoration may not be Regency-ready, but that can be changed.
I bought the pictured dress for $10 at Ross (yes, I love that place). It’s white, flowy and has the right waistline. I need to add sleeves of some kind and sew a ribbon over the sequins, but overall it’s a solid start.
Before I went dress-hunting, I spent a lot of time perusing Regency dresses in illustrations and museum archives.
And then there were the ones in the movies, of course.
I like the look of the ones in the 2005 Pride and Prejudice because they seemed easier to mimic.
Here are some modern dresses that might work as a Regency-ish starting point. I think the main thing to take away from these is that you can find cap-sleeves, long cuts and empire-waistlines in a lot of current designs. While you can easily buy these dresses online, I liked the ability to try one on so I could move around a lot in it, as I would at a real dance. I don’t know if all dresses are cut to move easily, so keep that in mind.
If you have the budget you can also check out Etsy for some LEGIT Regency dresses.
Of course, there’s always the chance that you’re a DIY wizard, so here are some links for those adventurous enough to make their own.
I also HIGHLY suggest going to thrift shops. They often have old wedding and bridesmaid’s dresses that could be a good base dress. Again, you can make your own, but I think it’s perfectly acceptable to take a modern dress and fix it up with ribbons and such to make it work.
If you’re lucky enough to live in a big area, there might be Regency societies that can help you out with clothing swaps or guidance for dress-shopping. A lot of times costume societies are very welcoming of people who don’t have the historically accurate costume, so don’t be afraid to approach them.