I am an Australian living in Japan.
Japan and Australia both are signatories to Unidroit, so an international will should be possible. I assume I need a lawyer for that. How can I find a lawyer to prepare an international will?
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The best way to find a lawyer as an expat in Japan (or elsewhere) is to ask other expats with the same background as you if they have hired a lawyer for a similar reason in the past. If so, and they were they satisfied with their work, could they put you in touch, etc.
If you don't have expat friends with a similar background who could provide solid leads, or you already have tried this angle and it wasn't sufficient, another good option always is a country-specific Chamber of Commerce.
For an Australian, in particular, that would be the Australian & New Zealand Chamber of Commerce in Japan. Attending one of their networking events would be a first step to finding an applicable attorney (and building your network in general). You also could refer to the Australian Embassy directory of lawyers by location, which provides a list of attorneys not only in major cities in Japan, but also in smaller ones, as well.