Just pull a new box from your class. Right click on it and search. Find the attack modifier deck and drag it out. I don't think there's a button to press to bring back a new one.

When I did that, I was able to find copies of the class-specific modifiers that you add to your deck based on perks, but I wasn't able to find just the default decks with the numbers in the corners. I ended up just importing the missing ones from a fresh game.

You can grab a new character box and use add player and a new deck will be generated. The base attack modifier deck is not generated until the character box has been added as a player.