Except for real mass-market boats I don't believe any of these can be bought "off the shelf" they are built to order by a local fabricator. This is done all the time, you just need to find one in your area that has a good reputation.
I don't think you'll find one ready made. You'll need to have it custom made. In south east Florida, these guys do a lot of that kind of work: https://www.pipewelders.com
Ask your boat yard if they can do it. If they can't, they'll know who can.