European data can be obtained from ECMWF at http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/ and they have an API https://software.ecmwf.int/wiki/display/WEBAPI/Python+ERA-interim+examples
Zhang, Liu, and Zhang (2018) share the data from their paper in this GitHub repository. From the README:
The CCSN dataset contains 2543 cloud images. According to the World
Meterological Organization’s genera-based classification
recommendation, we divide into 11 different categories： Ac, Sc, Ns,
Cu, Ci, Cc, Cb, As, Ct, Cs, St. It is worth noting ...