How to inform/attract more candidates for a postdoctoral position?
Possible outlets I can think of include:


Field-specific e-mail lists
Department email-list forwarded through collegues at other departments
Adverts inserted in talks/presentations held by people at your department (e.g. at invited talks, conferences etc.)
Job ad websites (national and global)
Advert at research-focused web communities/forums, such as Researchgate and Academia.edu, as well as LinkedIn.