How/Where can I get additional weather information for flight planning?
In Europe, several national meteorological services provide additional info (sometimes on a paid basis). Some other services cater e.g. especially to glider pilots. Examples:


http://www.flugwetter.de/ ($)
DWD - Deutscher Wetterdienst Luftfahrt  (free)
http://www.meteoschweiz.admin.ch/web/de/services/aviatik.html ($)
http://www.nzz.ch/wetter/thermikprognose/ (free)
http://wetter-jetzt.de/  ($)
...


There are also a number of private websites run by clubs and/or enthusiast pilots, e.g. http://www.schaenissoaring.ch/wetter_berichte.cfm?SelectedMenuID=154 or http://www.alpenstreckenflug.de/wettera.htm which offer excellent (curated) information, sometimes on very special conditions like mountain waves etc..