Any metalworking shop can make them up for you. Make a mock up out of cheap plywood, and they can cut the steel plate and weld it all up for you.
What youy then have to be very careful to do is to ensure the hull is strong enough to take the not inconsiderable twisting and thrust loads they can be subjected to when you take the ground - particularly accidentally! Usually this will involve building additional reinforcewment inside the hull to spread the load fairly back to the main keel, and out beyind the bilge plates so that if she gets pounded on the bottom they dont come up through the hull, and ensuring they can not be wrenched off if she is held on a bilge plate sideways against several knots of current!
But it is usually much better and easier to get a boat that already has them as the hull will have been designed with these loadings in mind.
-------------------- If you cant fix it, get a bigger hammer...