I think species such as red-necked wallabies and parma wallabies are common throughout Europe (especially the UK) - except in Norway where exotics are banned. From a bit of research and looking on selling websites, kangaroos are also quite common in America.
Right: specific species are common outside of Australasia. Others are not but appear sporadically. How easy/hard are they to obtain - for example the brushtails and ringtails, or the quolls.
One I forgot to mention is Sugar gliders. They are very common as pets and can be fed a commercial diet, fruit and mealworms. They are often kept in fairly small cages, however some keep them in aviaries. Sugar gliders cannot be litter trained either. Short-tailed opossums are also quite common in the pet trade and in America, I believe many people keep Virginia opossums. Brush tail possums are not very common (but they do exist in private collections) due to them not being rare in the pet trade and expensive, they also are very aggressive and have sharp claws. According to some accounts, they can be fairly gentle however now there are many bans on importing them, so it is extremely difficult to get hold of one. The enclosure size would have to be quite large and they typically eat leaves, bark and a few other foods when kept as pets. Ringtails can scratch and bite and I haven't seen them much in private collections, however I have noticed many people keep them in outdoor aviaries usually in petting zoos. They breed easily as well. Quolls are quite rare and I have not heard of any breeders or private collectors outside of Australia. They are very aggressive as well. Bettongs - mainly Brush-tailed bettongs - are fairly common in America and they are quite gentle, although sometimes destructive.
Shame no one exports the hopping mice though they are not marsupials: there seem to be no monongas in the UK thinking about it, if we are thinking of rodents kept as pets elsewhere but missing elsewhere. I started this thread because things like Dorcopsis pop up in zoos from private collections: it makes you wonder where.
A lot of exotic animal collections that are private will most likely have either Red-necked wallabies or either Red kangaroos or if lucky maybe some Eastern gray kangaroos. Like the previously mentioned posts, people also keep sugar gliders and to a lesser extent, Virginia and Short-tailed opossums and bettongs.
