Scholarly editions of music fall into two categories:
Monuments and historical sets, which are typically series devoted either to the music of a particular country or period in music history. They are usually classed in the M2s.
Collected works of individual composers which attempt to include every composition written by a composer. These are usually classed in the M3s.
The easiest way to find a particular piece in a scholarly edition is to consult Grove Music Online (in Oxford Music Online). Follow these steps:
Look up a composer and click on the tab for works, usually in the left sidebar.
Look for a note on scholarly editions at the beginning of the works list.
Notice how the editions are abbreviated and organized.
Works lists are usually organized by genre, so look for your piece within the appropriate section.
If no collected works edition exists for the composer, then scan the works list to see if individual works are included within a monument or historical set. The latter will be referred to by an abbreviation. To find the actual title, click on the linked abbreviation.