There are many timeseries datasets that are available for practice and experimentation. One of the most famous ones is the Air Passengers dataset, containing monthly number of air passengers from 1949 to 1960. Due to the nature of it, the data has cycles and trends as shown below.
Since the variance increases over time, we're going to almost always apply the preprocessing to make it a stationary timeseries. Below is the training data of air passengers after diff and yeo-johnson transformation.