hello i'm a historical fashion researcher and i have a lot of experience looking up things! this is a very widely experienced irritation and you're definitely not alone in this, but i am here to share everything i know!
so, ways to get around this:
turn off AI results. they're literally nonsense to us
don't use pinterest because the sources/provenance is often hard to trace
a standard internet search can be okay, but museum collections are the top tier (list of collections below this list)
instead of broad terms like victorian, regency, tudor, renaissance etc. try using the decade you're looking for. if you're not sure of what decade it is but have a vague image in your head, look on the fashion history timeline and just jump around until you find it. but even changing to e.g. 19th century will give better results than victorian
including terms like womenswear/menswear, daywear, formal wear, evening wear, court dress should increase the value of your search too
including "fashion plates" in your search can give you a nice impression of the intended silhouettes of the era. some of these might be a little stylised but will show you what was considered in vogue
for pre-fashion plate eras or things like makeup and styling, you'll have to look at portraiture or manuscripts. these are harder to actually find what you're looking for, but searching museum collections and limiting results to specific date ranges will be your friend
when looking at art, do bear in mind sometimes artists would paint fabric extra flow-y to show off their skills. it might not have been exactly like that in terms of fabric weight or drape. so, a pinch of salt required!
if you find something on image search where the provenance is dubious, reverse image search and you might find a source! i've been able to trace random pinterest images to real sources, but this does take a lot of time and effort and is often not worth the headache
some online resources and museum collections:
fashion history timeline is an invaluable resource if you're trying to get a feel for everything and should be your first port of call. it'll also link to good examples
the met has a vast number of extant examples of clothing, as well as fashion plates
costume institute fashion plates is a subcollection of the met for fashion plates (1800s-1922)
v&a also has many extant garments, fashion plates, and incredible articles on clothing and aesthetics. read the details of the objects because they'll often reveal a lot about the piece
lacma is good for C19th-20th pieces
nypl digital collection for photographs
national portrait gallery or similar for portraiture, or literally any museum in your country that has historical art
national museums scotland can be useful situationally but might be oddly specific
stout style history is a great collection for finding image references for fat people wearing historical clothes. survival bias of a lot of museum pieces tends towards smaller clothing that couldn't be repurposed, but this aims to counter that. it's not sortable, but is still a really nice resource
wikimedia commons is surprisingly handy! and the images, if you should need to link/repost them, are public domain
auction websites sound like a funny one to recommend. some won't have mannequins and some will. just look up historical garment auctions and you'll find some!
anyway, i hope this has been a good place to start for anyone interested! there are probably some i've missed because there are so many museums across the world and i don't know about all of them or can't remember them. but these are the ones i've used the most! (my specialisation/jobs i've had to research for have only really been in western fashion, so my resources reflect that)
dededos
stormborns
annaxmalina
snowghoul
yousharknotpass
origamiaround
