Although the black bolts are Stainless Steel
I tend to replace mine with Stainless Steel Button Head Allen key flange bolts
These have a washer built in so I don't need to use a black washer and a bolt
Most of them are the metric size M6 x 16 or 20mm in length
Most have been purchased via e bay and with free postage and in various quantities work out at quite a reasonable price
There are 2 types A2 and A4 Stainless Steel A4 being the marine grade
You can also find other types of Bolts, Washers & Nuts etc