Bladder ok for water, but not for waste
Your best source for a new waste holding tank is Ronco Plastics...they build TOP quality tanks for a very reasonable price and have more than 400 shapes and sizes to choose from, over a 100 of which are non-rectangular. Their catalog is on their website at http://www.ronco-plastics.com. They are a US mfr, whch means that shipping charges, including such things as "custom brokerage fees" can be a real killer. When I owned my company a lot of our customers who lived within an hour or two drive of the US border got US shipping addresses at places like Mailboxes Etc, or an independent who offered the same things, but at a lower price. Some even went in together with friends and dockmates, and took turns coming across to get what they'd ordered from West Marine and other US sources. It doesn't save any duty, but the amounts are usually low enough to avoid duty...and it saved 'em a fortune in "international" shipping charges, "customs brokerage fees etc.