South American is known for its alpacas. There are not a lot of other places in the world that have an abundance of alpacas like the countries in South America. Peru is probably the main if not only country that has alpaca rides due to it having so many of these animals for other reasons. Many shave them for their wool.
However, main tourist spots in Peru would have sites for alpaca rides. Since the Incas lived there in the mountains, this group of people created highways even though they lived in ancient times. Now, people would like to travel in the same ways and on the same route as the Incas. Many guides will take people through the Peruvian mountains on alpacas.
Riding alpacas probably isn't the smartest ideas for two reasons: they can be somewhat mean and will try to buck you off and the fact that an adult could seriously injure this animal. Alpacas may look similar to a llama but they are not the same size nor bred for the same amount of weight a llama can carry.
Truth be told, neither animal should really be carrying adult humans. The most an alpaca should carry is a small child or backpack. If you'd really like to allow your children to ride an alpaca, definitely consider bringing them to South America or alpaca farms in the United States. Other than that, maybe ride a donkey.