Okay, when I searched sex bolts it showed ones that weren't flat socket head style, more like a button head.

I only see them in stainless in the sizes and type that you're looking for.

https://www.mcmaster.com/sex-bolts/flat-head-binding-barrels-and-screws/

https://www.boltdepot.com/Sex_bolts_Socket_flat_head_Stainless_steel_18-8.aspx