Nordic folk music jam songs for harmonica: http://jamihuuliharppu.wordpress.com/

Tips, tricks and videos for studying overbend techniques: http://overblow.com/

David Barrett’s Bluesharmonica.com: http://www.bluesharmonica.com/

Site about advanced harmonica techniques:
https://en.wikibooks.org/wiki/Harmonica/Advance_techniques

Spare parts for harmonicas:

Hohner cs-shop: http://www.hohner-cshop.de/en/home/?cur=0&lang=1

Brendan Power: http://www.brendan-power.com/harmonicas-twin.php

Other links:

Online information package and playing guide for overtone flutes (aka itkähuilu, sälgflöjt, seljefløyte, pikkvile): http://www.overtoneflute.fi