Are you familiar with the Web Gallery of Art?
It's my favourite source for art sources; it may not be the most comprehensive anymore, or the best in terms of quality of pictures, but it's good for searching. You can choose type of art ("Portrait"!), era, and school (i.e. geography), and that makes it great for discovering relevant things.
I may search for the art and artists elsewhere after that sometimes, but I would not have discovered a lot without it.
This 1802 portrait of Heinrike Dannecker by Christian Gottlieb Schick is probably my absolute favourite, and I've been trying my hand at recreating her clothes. This study suggests it may be partial artistic licence, but I did manage. :D (I haven't completely figured out her chemise / shirt yet, though, and my hair does not do that.)
This portrait of Wilhelmine Cotta is also from 1802, and absolutely fascinating - she has no waistline! The closest to a chemise I've seen a chemise dress get. I'm thinking it could be similar to this one, but without the tie... (I really like that one, and am thinking of turning the pattern into the old red dress idea I mentioned in my Origin Story post, with sleeves closer to this painting's length.)
Also, I love her shoes. Blue and white stripes are a thing I have a thing for.
Her hairstyle looks more friendly to my type of hair. Although I don't have Regency bangs...
This 1810 portrait by Donát János is Central European (he was Hungarian) AND is the quintessential Little White Regency(-ish) dress. If I ever make a LWRD, I'll take my cues from this one; it's so quintessentially LWRD I think it can cover a good stretch of time just by a change of accessories, hairstyles etc.
Not this hairstyle, though. That's not me.