What service books are you referring to? The generic ones for the period, with either a sedan or coupe on the cover that are meant to keep a record of the service done on the car? These are relatively easy to find. If it is just for to make the set of books look complete, they are out there.
I took a look at the MB website as Hans mentioned, but did not see one specific for the 113. All the rest on the MB page referenced a particular model. The ones in the day, I believe, did not.
An unused original one would be quite a find.
Or a service manual for doing the work?