Wikicommons is a fantastic source, probably the most common one I dig things up in. Sometimes from an article on Wikipedia, sometimes just by searching something random on Wikicommons itself, like '1896 in warfare', and just browse from pic to pic, or category to category, until something interesting catches my eye.

rarehistoricalphotos.com is a good source of pre-curated interesting photos, but I try not to overuse it.

If a photo is mentioned in something I read, like an article or a book, sometimes I try to ford through the rapidly-enshittifying search engines of the modern day in the hopes of finding it. I come across some real niche Web 1.0 websites like that, still out there categorizing all the left-handed jugglers of Central Albania in the late 19th century, sort of energy.

A rare few I find on national government archives - those are a pain to look through, but you can find some real unknown gems. I found some fantastic photos of early 20th century Ethiopia on the Polish government's historical archives a while back - just the kind of connections you wouldn't expect to see.