I found the musical "flat" character for Microsoft Word in a font
titled "WP Iconic Symbols A".  With that font enabled, typing the
equal sign (=) character gives the "flat" symbol.  The sharp and
natural symbols can also be found in the WP Iconic Symbols A font
by typing < and >.

That font may have come from Word Perfect; I don't know if it is
present in Microsoft Word alone.  It's possible that I have that
font because I also have Word Perfect on the computer.

Fritz Gellerman