There are a few Stack Exchange sites you might want to check out:
Open Data: Open Data Stack Exchange is a question and answer site for developers and researchers interested in open data.
Data Science: Data Science Stack Exchange is a question and answer site for Data science professionals, Machine Learning specialists, and those interested in learning more about the field.
Cross Validated: Cross Validated is a question and answer site for people interested in statistics, machine learning, data analysis, data mining, and data visualization.
Kaggle also offers a bunch of example datasets and a bunch of real-world problems that can be used in a class.
But taking a step back, you might want to teach the meta-lesson of finding data in the real world. There are a ton of APIs and open-source data repos out there, and finding them is a very important skill by itself- plus it give students a way to actually use their skills after the class ends.
So instead of just giving your students a flat file, have them come up with their own. Maybe they use an API from a product they use, like Twitter, Spotify, or YouTube. If that's too advanced for the students to do, then maybe you spend a class coming up with the dataset together, with you writing the code and the students talking about what would be interesting to see.
Or just searching for "XYZ datasets" returns a ton of results. Have the students come up with datasets related to stuff they're interested in.