A quick look on Amazon shows a number of fairly recent collections of his work (which include introductions and analysis), plus some older books one can buy used. Among them is The Supernatural Tales of Fitz-James O'Brien, a two book set I bought used, and which I think may have everything —although by the time I bought the set, I think I'd located most of the stories online for free. I really would not call him neglected.
For other authors, I believe most of the books in Wordsworth Editions series Tales of Mystery and the Supernatural are pretty readily available. A lot of classic and lesser known stories by 19th and early 20th century authors brought back into the light during the last decade or so. But they are mostly single author collections, which doesn't give the same opportunities for discovering new authors while enjoying revisiting those one already knows. (But a variety of authors one may know from other types of fiction, for which they are these days more famous.)
For other authors, I believe most of the books in Wordsworth Editions series Tales of Mystery and the Supernatural are pretty readily available. A lot of classic and lesser known stories by 19th and early 20th century authors brought back into the light during the last decade or so. But they are mostly single author collections, which doesn't give the same opportunities for discovering new authors while enjoying revisiting those one already knows. (But a variety of authors one may know from other types of fiction, for which they are these days more famous.)