I inherited mine from my father when he died in the late 1970's. They are pretty cheaply made, but I found it to be amazingly accurate. I put a really cheap Simmons scope on it and it's kind of fun to shoot. I too experienced a broken firing pin one day while at the range. I got my replacement through Numrich Gun Parts in New York. There is also a guy in Alabama called Bob's Gun Parts that has Erma M1 carbine firing pins in stock. He advertises in Shotgun News.
I don't have nor would I know where to find a manual for the thing. They are easy to operate and fairly simple to tear-down and reassemle
There are 15 and 30 round mags available for them. Again, Bob's Gun Parts carries them, but they are really pricey for what you get.