I used to go to 2-3 military balls a year, and I frequently wear them for theatre. There is no worse misery than an uncomfortable gown in a formal setting, so I have some personal criteria for ball gowns.
These are all my personal preferences...
1). They should look timeless, or, barring that, 1930s or 1950s. A trendy gown is really going to limit the lifespan of wearability of a gown for me. And I can wear a period-looking dress for stage. So it earns its keep in my closet!
2). I have to be able to sit comfortably and eat a reasonable meal in it. This is non-negotiable for me!
3). I won’t rule out control top hosiery or light Spanx-Type stuff, but I prefer not to have to wear it.
4). The gown either has to fit a bra underneath, or have sewn-in cups and corset-seaming. Either way, the ‘girls’ have gotta be hoisted up and the weight distributed off my neck and shoulders.
5). It cannot look like a bridesmaid dress. I can’t quite specify... but I think we all know a bridesmaid dress when we see it. (That being said, a lot of bridesmaid dresses restyle well as Edwardian-look, so if you’re a Downton Abbey fan, maybe go check out bridal clearance)
IMPORTANT TIP: Life happens, and it is always easier to have a dress that’s a little big taken in, than it is to try and lose 5 pounds in a short period of time.
Honestly, my very best gowns are all thrifted or were bought off clearance racks. Textiles, beadwork, and craftsmanship were just better prior to 2000. Or at the very least, more affordable. You have some time to search - go dig around Nordstrom and Rack - but also hit the consignment and thrift stores in the more affluent neighborhoods where you are. I go by my local GW every week, and there’s a constant influx of beautiful gowns.
Oh... also... you could check with your local theatre or opera company and see if they have anything they can loan, rent, or sell you. People donate amazing things to local theatre all the time.