I replaced the transom shower on my 342 a few years back. I found the same one that was installed on my boat online. I can't remember what the web site was but I found it at several websites and went with the least expensive site that still seemed legit.
Any boating store, including online, will carry what you need. You just need to know the hole size for it to fit into. I replaced mine with something a little better last year.
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Boat Name: King Kong
"Boat + Water = Fun"
Dream 'Inn III -- 2008 400 Express