sounds like either a 3100 (.177 cal) or a 3120 (.22 cal) however a picture would certainly help in identification.
You should find the model number on the rear of the lower tube.
If it is missing the magazine tube the chances of finding a replacement is pretty slim. However, you still might want to check with the service shops on my repairs webpage.
getting the gun to shoot again should be hard, since the 3100/3120 have the same internal parts as the much more common 310/317/322 models.
The only manual for the 3100/3120 is a small parts list of the different parts used on these guns. Otherwise, the 310/317/322 manual is it.
See my book, The St Louis and Benjamin Air Rifle Cos. for a complete set of manuals for all of the Benjamin models.