Ebay, but its been a long time since I saw one offered there.
I'd suggest you take the boat by a local welder/metal fab shop and see if they will weld one up for you, using the boat and the existing corner for reference.
It may take a fairly long time to find one for sale.
You might get lucky and find a scrap yard or boat salvage place w one.
Welcome to iboats, nice glass Starcraft