One of my favorite sites is boltdepot.com. You can get about any diameter and length in various metals. I have used their stainless steel oval heads in trim areas with a little polishing. The neat thing is you can buy, by the piece without the typical 50 or 100 count box. Here is a link for #12-24 machine screws in slotted flat head steel.
https://www.boltdepot.com/Machine_sc...eel_12-24.aspx Back up a page and you can see many head types as well as materials they can supply.