Grady might be a good source to either buy it or find out who the manufacturer is. And then go to the manufacturer and find out if they either sell direct or can point you to a retailer.
There's a company called Taco Metals and they are often used by boat manufacturers for the supply of pieces like this. I don't know if they make what you need (or if they were the original fabricator for Grady), but you might try checking with them, too.