Three words. Gunbroker, Gunsamerica, and Gunsinternational.
What you are looking for is not an easy find. Most Model 12’s are either shot up, modified, or very high priced, especially for a gun that may be a hundred years old. It’s quite possible most guns you will look at will have all three features I’ve identified.
There are are also LOTS of variables. Chokes, ribs, and stocks can be a huge variant. Be prepared to pay $2,500 and up for a gun in very good or better condition. You might check with Indiana Gun Club. They sometimes have a Model 12 or two in inventory.