Try “Totally Stainless” located in Hiedlersburg, PA. They have virtually any type of slotted head screw you will need, along with all type washers, nuts etc. The prices are often less than what the hardware stores charge for their junk, much stronger and better quality. They generally can ship same day too. Luckily we are only about 15 minutes from them so I rarely even think about going to a hardware store. They have a website with their whole catalog to shop from as well.