Best genealogy option to find family member from Japan?

Tale as old as time... My grandfather was stationed in japan and we know he had a child there but nothing else really. He loved the woman very much but chose to stay with my grandmother. We found letters after he died but it's a dead end.

I see so many who test and find family so maybe that's an option for us. What would be the best one to find a relative in japan?

Thank you!