Kyle, got mine from McMasterCarr. Stainless -- some 316, some 304.
They also come in a T-nut form that I've epoxied into place.
Can't say they have exactly what you show...
Top Notch Fasteners has up to 1/4" bronze slotted FHMS.
And also bronze cup washers to match.
They have regular but no bronze special nuts.
316 T or Sex nuts should be fine with SB.
I guess you photo shows 1/4" MS?
not exactly... There is a taper that fits into the taper of the outboard mounted cleat. Frustrating that this piece of hardware apparently doesn't exist anymore! Thinking on what an alternative would be.