I assume you're talking about the step plates that are mounted on the cap rails at the bording gates. As I described in an early post, a number of years ago the founder of the Grand Banks owners forum arranged to have new step plates made by a foundry for those members who wanted to order them. The original plates are bronze, by the way, not brass.
So far as I know these step plates are no longer available new and have not been for many years. The broker who found our boat for us gave us a pair of new ones when we acquired the boat as a thank you for our business., but this was in 1998.
What we didn't have was the curved step plate that goes on the transom caprail at the transom gate. So we ordered one of those from the GB forum founder when he was having the new bronze plates made for the members who'd ordered them. Both the straight and curved plates were made using original plates for the patterns so they are identical to the originals.
But this was a one-time run at the foundry and only the number ordered were made.
The plates used on newer GBs are chrome-plated metal of some sort..
So as far as I know, unless one wants to contract with a foundry to have some custom made the best bet is to try to find some off an old GB that's being salvaged or parted out.