Here are a few more titles of songs that I couldn't find sheet music for—at least not online. They are all listed in the catalogue of the British Library, and some are also in other libraries.

MOTHER IS MY BEST GIRL NOW.
John McCale, etc.
New York : Spaulding & Gray, [1894]

MY BEST GIRL.
W. C. Robey.
Chicago : National Music Co, [1895]

A PICTURE OF MY BEST GIRL.
Charles A Moreland
New York : Howley, Haviland & Co, [1896]

MOTHER'S MY BEST GIRL
Charles Harvey.
[New York] : Hamilton S. Gordon, [1896.]

SHE'S MY BEST GIRL
Alfred Rawlings
London : Francis, Day & Hunter, 1903.

MY BEST GIRL.
Words by A. Seymour Brown. Music by Nat D Ayer
New York, etc : M. Witmark & Sons, [1909]

MY BEST GIRL IS MOTHER.
Clarence Vornkahl
New York : Globe Music Co, [1912]