To start with, an electrical outlet box is the protective casing in which the circuitry for an electrical outlet is stored within.
Behind each wall where there's an electric socket there's almost always one of these devices and as such they're incredibly common across the world. They can be made out of a variety of materials but they're usually made out of iron / steel or plastics.
They're made without any particular durable qualities, mainly because they're inside a wall which offers the majority of the protection that it needs. As you can imagine there are a vast amount of different varieties and sizes. Just as you get double sockets you can get double electrical outlet boxes and they're designed for any number of amounts. Usually you just tend to see sizes of up to two but you can easily find larger ones – or of course just place them next to each other.
Depending on the country you get different designs and sizes because of course from country to country the socket tends to change in design. For example American, English and European sockets are all very different from each other – but Malaysian and English ones are the same however.
This means that if you want to buy electrical outlet boxes online you really have to check whether it's the right type. They can sometimes be very non-descript so the best thing to do really is go down to your local hardware store. Here they may charge slightly more, but it won't be a lot considering the peace of mind you'll have!