Ebay has been my best source for manuals. I have used autolit, both through ebay and directly and they have been reputable to deal with. I have also used toad hall and books4 cars, also good to deal with.
There are a number of aftermarket manuals out there, The one by Ken(I believe) Ball is gotten decent reviews on the site. Glenn's is another. A lot of these books have the same content, as it all is originating mostly from the same source, MB.
I have had good luck tracking down a lot of the original pamphlets, maintenance books, etc. THere is a post on the site, with picture that shows a very complete set up for the 113's (manual, parts book, A/C, seat belts, spare tire card, dealer directory, etc>)
I would keep a standard query on ebay and just be patient.
Jonny B
1967 250SL Auto