As for the location of the power steering reservoir, picture attached. RHD, I suppose, no difference in location versus LHD, but just to make it in order.
Was your car equipped with power steering?
If you meant where to source the power steering reservoir - that is a tougher question. I have not seen it offered by the usual suspects. I would try: there is a chapter here on the Forum Wanted To Buy and I would try on ebay.
If you are starting your journey with W113, it may happen you will love it and stay for longer. Honest advise: become a Full Member. The fee is symbolic while the membership will give you access to Technical Manual, which will answer for you hundreds of questions you are going to ask.