The Power Pad? I LIVED it, man! My parents fought to find an NES for me the first Christmas they were availabe, way back in the day. Of course, I was about 4, so I don't really remember all the details. My NES came with the first legal American multicart, with SMB, Duck Hunt, and World Class Track Meet. I had the Zapper and the Power Pad and yes, we would just get on our knees and use our hands. Those were the good old days... my house was broken into in 1992 and all my videogames were stolen, including the Power Pad. As a 3rd grader, I was ticked.
Anyway, the Power Pad is still available at used video game stores. It was fairly widespread.