To see wallabies, there are plenty of places you could go. One of the best bets to see a wallaby in a controlled, safe environment, look up zoos that have wallabies among their exhibits. If you want to see one in its wild habitat, you should go to Australia, New Zealand, or New Guinea. Any of the islands around there could also work, but there are so many of them it can be difficult to find the right one.
The best bet to see a wallaby is probably going to a zoo, as hard to swallow as that might be for someone who would rather see a wild wallaby. The Detroit Zoo, Smithsonian's National Zoo, and the Toronto Zoo are examples of zoos that have had a wallaby in the past; always check to make sure the exhibits are open when planning your trip.