You can always try "nature jobs" or similar websites. When I was looking for PhDs, I frequently looked on nature jobs and found that most of the new PhD positions in natural sciences were advertised there. I think however that this will be difficult. Majority of post docs are either paid by a fellowship or they got the position over connections. For one of the open post doc positions in my lab they had over 300 applicants and I was told that this is pretty much standard today. Some positions are only advertised because they are obliged to advertise it, even though they already have someone for the position. I guess you need an opening in more or less exactly the specialized field you did your PhD in to be competitive. But as applying doesn't cost anything but time, you can always try. Maybe your supervisor can pull some strings or knows some people in your field that might need a postdoc in the near future. That's probably the most promising way to find a position.
Good luck!