Yes they are, STI covers both road and MTB, it stand for Shimano Total Integration, and covers all combined shifter / brake levers, STI was printed on MTB units from the early 90's (Rapidfire era), although now the term is primarily used for road shifters now.

The Shifters the OP has look to be from the Rapidfire era (200GS?) / 1990 ish, for spares, this is long after Shimano has stopped supporting them, so either co-op/LBS with good spares from broken parts, a current barrel adjuster should have the same thread, and there are plenty of non shimano options, check out a search for brake barrel adjuster on ebay.