Another option is the Climate Research Unit (CRU) at the University of East Anglia.
High-resolution gridded datasets (and derived products)
Cloud cover (cld) is provided as a percent. Retrieving data requires registration.
An example dataset:
A gridded time-series dataset. This version, released 4 July 2014, covers the period 1901-2013
Coverage: All land areas (excluding Antarctica) at 0.5° resolution
Variables: pre, tmp, tmx, tmn, dtr, vap, cld, wet, frs, pet
(You can also access this data from Google Earth)
