Might be a long shot but have you tried going to a cement yard and asking for their leftovers? Out here when the trucks come back with a bit left over they put it in a mold and make big blocks out of them. Some are upto about 2000kg. They dont have to be perfect or any set shape as long as its a bit porous so the tiny life can cling on at the start and theres hidey holes for the smaller fish things will start to happen. Or theres always the option of a wreck. Even if its not a ship. Get an old stripped car, bus or truck and clean it out. Weld the doors shut, take them off or whatever you need to make it safe. Remove the driveline and fuel tank. Its a small start if you use a car but if its the cheapest option maybe you can get more of them. Even wrecked cars and ones that are chopped in half will do. They get pulled out of the river close to me all the time and some have only been there for days and already show signs of life. You could even try the local police for cars that have been impounded and never claimed. The army might have a few old tanks even. They might donate them for a decent cause. But you never know till you ask someone. Hit up local businesses that will also benefit from more people come to dive it. Just dont take no for an answer.

Matt