Three other options to consider: check out Thoroughbred Miniatures 15 mm ships, check in with the Virtual Armchair General and his connections with Houston Ships (custom, so maybe hard to find), or - depends on where you live, I suppose - try and find a 'mantle piece' ship and waterline it. Here in California both Ross's (sometimes) and Hobby Lobby (more often - but more expensive) sometimes have ships that might work.
You have set yourself a challenge from the images I saw when googling: images of HMS Gannett.
Good luck.