Personally, many scholarly societies maintain websites that provide information about available postdocs. But I would say that this approach should come second to another, which is much more successful. I have found that the best approach is to find out in the literature who works in areas that you would like to do your postdoc on, read their articles, and then contact them directly, via a well-crafted e-mail (without any English mistake!!!), to let them know that you are very much interested in their research, that you would like to work with them, and that you are available for a postdoc or any other position, if they have funding for you. Believe me, many professors globally love to have applicants who are genuinely interested in what they are doing, and from my experience, they are willing to move mountains to attract them to their labs. Motivation is half the battle in research.