I'm looking for a reference that gives some information quantifying sources of mutations that occur within cells. These can be general, but I'm interested in some percentage of mutations caused by external vs internal sources (like mutagens vs replication errors).
An extensive collection of the various mutation rates (from which you could reverse-engineer their fraction on the total share of mutations):
I actually found this in the following paper:
This group shows estimated governing forces in mutation incidence by tissue types.