One of the most important things when looking for a shipping container manufacturer is to make sure they construct to ISO standards, this is not to do with safety but more the fact it is unlikely to be moveable by freight on sea or land as the docks and hauliers will reject it due to being awkward to move or in fact not movable by cranes adapted to shipping container usage (they need the shipping container mountings in specific places). As well as this you also need to work into the fact your designs would also be more difficult for your construction project with it not being standardised sizes.
One of the best places to do a search for shipping containers is on globalspec.com an engineers search engine although the information is biast towards the American market. Either way I know many of the readers here are from the U.S. and its a good place to start as you can find new, second user as well as some lease opportunities.
The National Portable Container Association also offers a list but its mainly for leasing,renting or arranging shipping of shipping containers. Still a handy source for advice though.
When looking at the suppliers be aware of your distance from them as getting someone as close to your location is possible when looking to locate a shipping container manufacturer will save you on transportation costs that could quickly add up over long distances.
But also when checking through the manufacturers be aware also that many specialise in specific units e.g. refrigeration as well as specific dimensions. This is often geared towards they’re larger customers as well as the equipment and locations to suit they’re needs. So specify what you want as well as check they are able to accommodate the shipping container units your looking for. Sizes generally come in 20ft and 40ft but companies may offer variants depending on the order and client. They should always be waterproof,rustproof and coated with a good quality long lasting marine paint. Inspect other units they have in process to assess the quality of the welds as well as the other materials being used in the construction including reading up on the paint they use to check the manufacturers guarantee and recommended uses.
One of the best shipping container manufacturers are likely to be carrying out military contracts where specifications are much higher than standard usage. Seeking out such a company is likely to guarantee a high quality product every time. Around this blog you will also find “pre manufactured” units being sold which are from all sorts of markets from the garden office to completely off grid housing and emergency relief. All of these companies in some capacity will be able to provide a shipping container home in some form and maybe worth searching the ones in your area even if your looking to build a shipping container home yourself, as they can no doubt provide some helpful tips and advice.