The larger marine chandlers should carry these items. You can (in the USA) purchase such items online at a variety of places. Manufacturers of good bronze thru-hulls and seacocks include US companies Buck Algonquin, Perko, Groco, among others. You can google these companies (add the words seacock or thru hull and you'll find online vendors with their products, too) and download .pdf catalogs from many of the manufacturers and then you'll be more likely to know what you're asking for. Some manifolds are custom ... the water tank manufacturers may be helpful in that regard. You can, for fresh water, use plumbing fittings in PEX for your needs and those can typically be found at a good plumbing supply place. Your local marine chandler should have bronze (and brass) pipe nipples, elbows, unions, etc that would allow you to build up a fitting that suits your need perfectly as well.
We're in the process of putting together our raw water system right now and its looking like a bronze animal
After the seacock, the 2-1/4" pipe comes in to a huge sea-strainer and then splits off to give 1-1/2" water to the engine and 1-1/4" water to the gen-set as well as something to the head (can't recall what) and 3/4" piping to the deck and anchor wash down system. All of these splits have their own valves and back-flow preventers so...its a lot of fittings in a row. My hubby is building a nice cabinet around the manifold on the bulkhead it resides on.
Our fresh water system isn't quite so complicated, thank goodness.
Best of luck to you! in your search.