Stores for plus size?

I’m having a bit of a problem when it comes to shopping more formal, but still chic.

I’m size 26, 2X/3X and everything I find is totally outdated in a sense that my grandmother wouldn’t wear it. And I know of Torrid, Catherine’s and SWAK, but those just aren’t as formal as I need.

Specifically I’m looking for a formal gown/dress for an event one could compare to prom.

Any ideas?