Bladders can be a good choice for water, but not for sewage for several reasons: they're highly prone to blow out a fitting at the first sign of a clog or a blocked vent (USCG regs require ALL waste tanks, rigid or flexible, to be vented to the outside of the boat)...it's almost impossible to secure a bladder to totally eliminate chafing /leaking...and even the best ones only have about a 15 year lifespan. A good quality rigid plastic tank will last as long as the boat provided it's not abused.
Ronco Plastics
Ronco Plastics marine Tanks is your best source for a tank. They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with.
When looking at drawings it's important to know that there is no top or bottom until the fittings go in, and YOU decide where they go. So rotate, flip in any orientation to find the tank that fit your space. Yes, they do have at least a couple of bow-shaped tanks and a whole bunch of "wedge" (triangular) shaped tanks to lie along the hull. Fwiw, Ronco has been the OEM supplier to most of the high-end boat builders for decades.
I'll be glad to help you with the details of selecting the best location for the tank, locating the fittings on the tank and plumbing runs.
--Peggie