Well, that depends on the scale you're working on and the data processing technologies you favour. The surface temperature data can be obtained through:
Copernicus global products https://land.copernicus.eu/global/products/lst
NASA global products https://lpdaac.usgs.gov/products/mod11c3v006/
or you can just write a script in Google Earth Engine, which has a tonne of global temperature products available (getting the mean values is described in GEE documentation) https://developers.google.com/earth-engine/datasets/tags/surface-temperature