or someplace locally or on the net that sells autobody supplies. this is the first google that came up
https://autofastenersandclips.com/co...ons/body-bolts
personally I find the black bolts last a little while and then start to show rust.
some fasteners have a built in washer that spins freely so it doesn't take paint off. some have a flange head so it looks like a washer but is part of the bolt so it spins with the bolt and digs into the finish when it tightens up. then there are some with a plastic washer to stop dissimilar metals from corroding. these work well on nice paint as well.
I recommend getting a tap of the correct thread and running it through all the holes to clean crud out and freshen up the threads. then use antisieze or lubricant on the threads so they don't rust. in some places I have used aviation form a gasket on the threads of screws etc where they run through a freshly drilled hole etc. that stops any ait from reaching the bare metal so the hole doesn't rust. it comes in a small can with a brush inside the lid or sometimes you can find it in a tube.it doesn't harden so stays pliable and wont melt out on a hot day. not easy to get off your hands though.
https://www.permatex.com/products/ga.../?locale=en_us