Potbelly Sandwich Works make a lot of sandwiches and are not likely to publish their "recipes", but if you buy one, you should be able to reverse engineer it pretty easily. I came across a nice recipe for meatballs in marinara sauce, apparently based on their recipe , here: Pardon My Crumbs: How To Make The Perfect Meatball Sandwich
It came out pretty well, and I had it with pasta, too.
There is an old Time Life book in the Good Cook series titled Snacks and Sandwiches
, and Amazon has it used for $4.00, including postage.
Mostly, though, I suggest that you buy some good bread, whip up a batch of Mayo, and have at it with your favorite fillings.