My container requirement was purely as a great big stuff-swallowing box. Santa, though, might prefer all sorts of the more fancy options and they are available as offices with windows, or even clad in wood for that North Pole log cabin look.
So where can you plonk your container? Well, the good news is that they are regarded as temporary structures. Just like caravans, they can be parked on your property and you should not need any planning permission. However, it’s wise to go and see your local planning office first, just in case. They may have restrictions when it comes to location, especially if you are in an area of natural beauty. There would be issues if you decided to start stacking them. It makes sense not to cover any mains drain access, or have power cables directly overhead.
If you have hard standing, then provided it can take the weight (a 20ft container is 2015kg and I know because it is spray painted on the door), it should be fine. Otherwise you need to prepare the ground, at least flatten it and put gravel down for drainage. What I did for my container was use a mini digger to remove the tree stumps, get a couple of railway sleepers and bed those in concrete so it could sit on them front and rear. Concrete blocks are another option. Just ensure that everything is level.
It is possible to buy a second-hand container unseen from an auction site. Private sellers might also have them in industrial yards or their gardens. Buying them is easy, but arranging transportation is less so. At that point, it starts to become rather complicated and expensive. Better to go to a specialist, who will guarantee that the container is watertight for at least 12 months and organise delivery.