One of the great resources for researchers of the nineteenth and twentieth centuries is the ever-increasing availability of journals and periodicals. For a long time, these were only really available in university and large research libraries, and in the cases of many of the more obscure and evanescent journals, only a few libraries held these. This situation was eased a little by reprints, but not again some of the more obscure articles were not included.
The invaluable companion for using these resources is a good bibliography, such as Allen Wade's bibliography for Yeats's own writings, or K. P. S. Jochum's bibliography for Yeats's and for others' writings about Yeats, such as reviews and interviews. The Variorum Edition of the Poems also gives the first publication place, which was often a journal.
I will start with a general portal, The Modernist Journals Project—modjourn.org— which provides "high-quality digital scans and metadata of periodicals published from 1890 to 1922". There are too many titles to list, but it's definitely a useful starting point for those who are looking for poems, articles, or essays that are part of Yeats's mainstream writing in their original state and context (I'll look at the resources for his more fringe interests later).
As a simple example here, I give Yeats's poem "He mourns for the Change that has come upon him and his Beloved, and longs for the End of the World" (Variorum Poems 153). The apparatus tells us that it was first published in The Dome in June 1897, before appearing in book form in The Wind Among the Reeds (catalogued as 11 at the front of the book). (As it also indicates, the titles were different in each case: "The Desire of Man and of Woman" in The Dome and "Mongan laments the Change that has come upon him and his Beloved"in The Wind Among the Reeds).
Going to the Modernist Journals Project and going through the alphabetical listing:
The whole issue can be downloaded in PDF from this page (see "Download Issue Data" on the left)—about 30 MB in this case.
Though it started in 2011, the project is very much ongoing and the site is still under continuing construction—there are places with "Lorem ipsum" dummy text and blank images. It offers useful tools that are well worth exploring, but it also rewards browsing and brings up plenty of unexpected insights into the period.
