I remember having seen a few sets of a WW2 German marching band. I don't remember the manufacturer (best guess is Andrea Miniatures or SOL Model).
There is an Italian company that makes figures of the Carabinieri (police) band, but they are all standing.
In both cases you'll have a hard time converting 90-100 figures to match your purpose. One year might not be enough.
Consider buying one marching figure, convert it, make a mold, and cast the other 89-99. Same thing with the instruments make one, make mold, cast the other ones.