The Siam Repository, published by the Baptist missionary Samuel J. Smith, is one of the key sources for earlier church history in Siam. According to Patricia M. Herbert and Anthony Milner, South-East Asia: Languages and Literatures : a Select Guide (Honolulu: University of Hawaii Press, 1989), page 30, it was published from 1869 to 1874. Most of the volumes are available online on the Google Books, Internet Archives, or the Hathitrust websites. It does, however, take some patience to sort out the results of a Google search of "Siam Repository." That is pretty much the story for searching any of the 19th century missionary serials.
It turns out that all five volumes of the Siam Repository are also available in hard copy and PDF formats through Rare Book Clubs (here) for a charge, of course. Purchasers are warned, "Books marked 'PDF/scan' may have notations, faded type, yellowed paper, missing or skewed pages, etc. Paperbacks marked 'OCR' may have numerous typos, missing text, with no illustrations or indexes." The PDFs can be downloaded from the book club website, but hard copies have to be purchased through other sellers. They are available at Amazon (here). The hard copies are not reprints. The originals were scanned, and anyone who has used scanned material knows what a mess the results can be. There are also two or more versions for some volumes, and it is not clear what the differences in the versions are.
In any event, chalk the availability of the Siam Repository in various forms and formats as one more example of how Internet has radically transformed the study of Thai church and missions history. It has made a great variety of 19th and early 20th century sources available online. While finding what is on the Web is a real hassle, the fact is that large amounts of published historical material are there waiting to be found—and when found then always immediately available thereafter.