Where can you find information on the value of a 1913 J Stevens 12 gauge shotgun?

With that version of the Stevens name, it would have been made after 1920 when the company became a division of Savage Arms and 1948 when the name was changed to simply "Stevens". As you said, it is a good, dependable gun, but it is not a collectable, so the value lies in it's utility. In most parts of the country it would retail for about $250 in NRA Good to Very Good condition.