In addition to the UCI Repository and Kaggle, I recommend the following :
1- The dataset available at the SPMF (Sequential Pattern Mining Framework) which is accessible from here [http://www.philippe-fournier-viger.com/spmf/index.php?link=datasets.php]
2- A collection of " Awesome Public Datasets" available in GitHub and can be accessed from here [https://github.com/awesomedata/awesome-public-datasets].
3- Datasets from Academic Torrents. Which is available here [http://academictorrents.com/browse.php?cat=6]
4- My answer to a related question in Quora [https://www.quora.com/Is-there-any-C-code-for-the-Apriori-algorithm/answer/Ibrahim-Musa-19].