The song you are talking about is called "Humphrey Hop". It is availible on the CD "Classic Disney: 60 Years of Musical Magic, Volume 5" (Orange cover). It's not the version from the cartoon, but a cover version of the song (it's still nice to listen too though). I don't know if the cartoon is availible on video or not, but it may have been availible on the earlier compilation videos from the 80's...not sure.
I remember seeing that cartoon ("In the Bag" 1956) all the time on TV when I was growing up though and it was one of my favorites. Humphrey was always so pathetic you couldn't help but love him! He sure did go to great lengths to get his dinner!
Ben C.