I have to ask exactly what you mean by "poor". By the period you're speaking about, photography had become fairly common, and many of the well-dressed photos you come across are likely to include lower-middle class people who are simply wearing their better clothes for the occasion, many of which might resemble the men's clothing advertisements you found on Google. I have family photos that show the same people dressed up in the studio and dressed down to work on the farm. So the question isn't so much "what did the rest of us wear?" but probably "what did working people wear on an everyday basis?"

In all honesty, clothes that weren't much different from a white shirt and jeans. For men, dark trousers in a fabric that would wear well, like a heavy cotton or wool twill, with either a plain white shirt with a detachable collar (with or without the collar itself, depending on the kind of work) or, if a really physical laborer, a white jersey knit undershirt. For women, either a dark, hard-wearing skirt with a white shirtwaist or a dress made of printed cotton. From about 1910 forward dresses tended to be made all in one piece, but before then they were more commonly two, and a woman might have paired the bodice from a worn-out skirt with a different skirt, or vice versa.

Here's a great example of a laborer in an undershirt and dark trousers. And here's what a printed cotton dress for working outside might look like, while these women are dressed for working in a laundry. This whole collection of early 20th century photos of African-American life have very good examples of everyday working dress.