Since my last update, I tried to find sites or specialized shops where I could buy a RAC adapter but failed miserably. I then turned my attention to second hand websites like Ebay or Yahoo auction but also  classified ads sites and I've been checking them everyday for the last month.

There is one brand new RAC adapter on sale on Ebay France for something like 110 euros but doesn't seem possible to have it send to Japan...

I reached to people around me last week and a former work acquaintance put me in contact with someone working in a small company that provides LED lighting fixtures and air conditioner systems to corporates. The quotation he provided me was pretty expensive: about 25000 yens.

I will try to contact that French guy on Ebay, also try to reach to Hitachi directly and see from there.