Crafting Shotgun Shells is one of the most lucrative uses of the Fabricator in Prey, so long as you have the plans.
Fortunately, these plans can be found very easily in the Security Office of the Talos I Lobby. Contrary to popular reports, you do not need to get the Security Office Keycard from I.T. Security Department (which is populated by two phantoms, I might add), in order to get into the Security Office.
There is an alternate path you can find by climbing the pipes on the left-hand side of the corridor leading down to the Security Office. Simply climb those pipes, open a maintenance hatch, and drop down into the office.
Once you drop down, turn 180 degrees and either hack into or use this code on the safe you see. Inside the safe is the Fabrication Plan for Shotgun Shells. Never run out of ammo again.