The Swiss Railway site at http://www.sbb.ch/ shows most connections for train, bus and ship
The Bern - Lotschberg - Simplon (BLS) railway is the main access to Kandersteg. See here http://www.bls.ch/e/bahn/fahrplan.php for relevant BLS timetables. http://www.fahrplanfelder.ch/bav_assets/files/2010/330.pdf takes you to the 2010 timetable for service 330 from Spiez through Mülenen (for the Niesen), Reichenbach (for Kiental and Griesalp), Frutigen (for Adelboden), Kandersteg, Goppenstein (for the Lötschental), Hohten, Ausserberg, Eggerberg, Lalden (for the Höhenweg Südrampe) and Brig . Timetables tend to change once a year in December.
The main site to get Post Bus information is http://www.fahrplanfelder.ch/ with a search box to access any relevant timetable. The Post Bus driver normally has timetables available on the bus. Here are various 2010 timetables:
If you go over the Golitschenpass to the Elsigenalp and take the Panoramaweg to Frutigen, you can catch the BLS bus from Frutigen to Kandersteg at the Rybrügg bus stop by the Tellenburg viaduct, rather than walking all the way in to Frutigen. The bus is on the standard BLS 330 timetable.