One of the early 1990's festivals had a ton of them. It really was overwhelming but by the next festival, a lot of those pictures, as well as quite a few of the dealers, were nowhere to be seen.
A lot of my DS pictures were purchased from photo dealers. I got a lot of stuff from Hollywood Book and Poster on Hollywood Boulevard. If they didn't have a particular picture in stock, they would print it up from the negative and mail it to you. I saw Jim Pierson's name all over the files. Unfortunately, Hollywood Book and Poster, which was one of the last of its kind, closed its doors in 2015.
Jerry Ohlinger's in Manhattan was another great source. They had five huge books of DS related photos, the original series, the films, the NBC series and convention photos. Of particular amusement was a rather unflattering picture of TLATKLS, which had a note attached to it saying: Will not sign this.
There are other outlets for unusual pictures. Getty images is one, if you don't mind the watermark.
Barbara Steele usually has a few DS shots available but you have to ask for them, as she doesn't always have them on display at her table.