I always had good luck at True Value Hardware Stores. There one of the very last places that have a million and one things in little drawers that you can spend a couple hours roaming thru until you find just the parts you need .
Those are most likely METRIC. I'd have to take a look at mine and I don't think I'll be able to get to it soon. As not to alarm anyone, I would disassemble the gun down to the receiver and just bring that into the hardware store. They may be a flat head screw (ie: goes into a conical seat) but don't quote me on that. Does Mossberg not have them?
Whatever you do, realize that the screws protrude into the receiver and if they are too long, they can interfere with/damage the bolt! Some trimming may be necessary unless you can find some screws that fit "just right". Too short and you may strip out the threads in the receiver... Good Luck and Welcome Aboard!