A few years ago this kind of houseplant would have been relatively hard to come by. It would have fallen into the class of rare plants and you would have struggled to find it in any local shops.
However, due to its increasing popularity, you're now able to find Begonia Maculatas in all kind of garden centres and local plant shops. Call the plant shops in your area to see if they have it in stock. If they don't then don't worry! They are plenty of places online where you can purchase this plant.
UK
Bears and Daisies
Crocus
USA
Bakker