Reference for unpublished but quoted manuscript
I assume you've looked online, though I'd be amazed if a 1980s master's thesis was online.

Ask one of the authors: they should have read the thesis before citing it, which suggests that at least one of them has access to a copy or, at least, had access at the time they wrote the paper. If that fails, try asking your university library.

Also, although the citation doesn't mention it, be prepared for the thesis to be in Russian.