Option 1: The fastest and most expensive approach is to buy a pre-built fiberglass hull from someone. This includes the makers who build for this hobby as well as other model makers. Most of the other model makers build lighter and with more detail which is pointless when you start flinging bb's at the ships. Here is a link the SCRAP site which has a current listing of known makers and the ships they are currently offering. A couple of thoughts. Large hulls are not cheap to ship (particularly from Australia). Don't buy a hull from one source and a deck kit from another, they may be working from significanty different plans.
Option 2: Hulls can be made out of 1/4 inch marine grade plywood. This is much cheaper but expect to put about 30 hours into building the hull (figure 40-60 hours for everything else) There are some beautiful hulls out there that were built this way. This is probably the cheapest - assuming you have lots of spare time and good woodworking skills.
Option 3: A one-off fiberglass hull can be built via a couple of approaches. One is to start with the above wooden hull, or a lighter version purpose built. The hull is then coated with a couple of layers of fiberglass. If a lighter frame is built, it may be completely removed after the fiberglass layers are strong enough to support themselves. This is somewhere in between without going through the full process of building a plug and mold.
There are some good reasons not to buy a pre-built fiberglass hull. a) There is no fiberglass hull available for the ship you want to build. b) you like working in wood and have lots of time and some experience.
There are some bad reasons not to buy a pre-built fiberglass hull. a) it is cheaper than buying some wood. (What is the cost of your time?) (fiberglass lasts longer than wood and requires less maintenance during it's lifetime)