Have you tried the http://www.worldclim.org ? The 30'' datasets corespnd to 1km on Equator.
There are some precipitation variables there (Annual Precipitation, Precipitation of Wettest Month, Precipitation of Wettest Quarter, and so on) but if you look on the databases, you could find the average monthly precipitation as well
These are not on satellite-based measurements but based on (a few) point measurement on the ground interpolated by lat, long and elevation and therefore of unknown accuracy.
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