Well, there are no leads after 291 because there were no photo galleries until the 1950s when Limelight, which was also a coffee shop, owned by Helen Gee, was in New York. Over the years, there were a number of galleries that had shows of photography, but strictly speaking, they were not photography galleries. Even 291 was not a "photography" gallery. Other art was shown there as well. 291 became the "Little Galleries of the Photo Secession" which showed photography, certainly, and also showed other "advanced" art as well, so strictly speaking, even 291 was not a "photography" gallery.
Carl Siembab gallery in Boston beginning in the early 60s (and maybe in the late fifties). Focus gallery in San Francisco owned by--her name is on the tip of my tongue, but eludes me. And there were a couple of galleries in the early 60s in New York. Norbert (Kleber?) had a gallery, downstairs and I believe he also sold camera equipment, I believe. And there was one other as well.
But the first truly successful modern gallery was the Witkin Gallery beginning in 1968, I believe--maybe beginning in late 1967--I'd have to look it up. Then came Tom Halsted's 831 Gallery in Birmingham, Michigan. This was really the second gallery. (I had a show there in 1970, a two-person show with Brett Weston and I even sold a print--for $25.) And then came Light Gallery in New York and Harry Lunn in Washington, but at first he sold other things besides photography. The history of all these things can be read in Jake Deschinís newsletters if you can find them. Jake was an old-timer--at one time he wrote for Pop Photo or Modern and in his newsletter he reported on what was going on. You should be able to find them at the New York Public Library, I would think.
The Witkin Gallery and Light Gallery were very successful and were a big part of starting the gallery "scene."
Beginning in 1975 other galleries started to spring up around the country. G. Ray Hawkins in LA opened in March. Maggie Weston in August opened her gallery in Carmel. And before then, the Visual Studies Workshop in Rochester had a gallery.
The Ansel Adams gallery had been ongoing in Yosemite for some time, but it almost does not count becasue for the longest time they only showed Ansel's work. BY 1975, I believe they were showing others as well.
Well, that's a beginning. Where to get information? Good question. I'd ask, politely, a few of the (now) old-timers.