You're both in an out of luck here.

The most straightforward way to access a range of these documents is a book called Spain Betrayed, edited by Ronald Radosh, Mary Habeck and Grigory Sevostianov. This is a profoundly strange book in many ways - it consists of a wide range of translated Soviet documents relating to Spain that became available in Russian archives in the 1990s, interwoven into a stream of editorialising that often bears relatively little resemblance to the source material being quoted. As the name suggests, the editors are firmly of the belief that Soviet intervention in Spain was entirely malicious, an absolute conclusion that is not really supported by the bulk of subsequent historical work utilising the same documents (and more). As such, it pays to be critical - the translated documents are hardly a random sample, and the editorial narrative far from balanced. Nonetheless, there's a lot of interesting documents in there, and to my knowledge they've all been accurately translated and sourced.

If you wanted to get really stuck in, there's usually a way to get your hands on relevant documents directly. A Russian Federal Archives project has digitised a huge amount of archival material from the period, including large chunks of the Comintern archives, one of the key agencies involved in Soviet intervention. Reddit's spam filter has a habit of blocking a direct link to the site, but if you google "sovdoc" it should be the top result. The site itself is in Russian (but presumably if you're after original Soviet documents that's not an issue for you) and a pain to navigate, but there's a huge amount of original source material on Spain in there. The catch is that the archive browser has been down for the last few weeks - the site reliability has always been patchy, but it seems to be completely broken just now. It may reappear tomorrow, or next year, or never.