The only place I've seen these is the stainless steel ones from Herm. I'd be fine using a just a new bolt, but the original style would be cool to have.
You can buy them new from vendors but the bolt head is flatter (not as tall) as the original. Herm's appears to be the correct size.
You should also consider Harry Sheets as he may be collecting them with all his other parts. Remember that with it you'll have a flat washer and a wavy washer that goes with it.
I'm not 100% sure, but believe the wavy washer goes between the frame arm and the bracket.
I will check with Mid West. I haven't dealt with them yet.