I need to find translators who can read business documents in Chinese
and Japanese (2 separate people). This is an ongoing job not a one-off
contract.

I need to post an advert for this role, in English. What are some
online recruitment sites, message boards and communities where I can
do so? (the equivalents of Monster, Craigslist and the WSJ).

Use the Google AdWords? Targeted to people who can only read in
Japanese and/or Chinese.

I'd rather say, you seek for a translator community board, mailing
list, because that's where the real people go. Not the recruitment
sites, too many people will overlook your ads.