Could you help me finding a site to look up my family history in Karstula, Finland?

You could try the site of the Finnish Genealogical Society at http://www.genealogia.fi/indexe.htm . You could also contact The Karstula Parish (usually the parish archives are the best source of information for genealogical studies), contact details at http://www.evl.fi/ (unfortunately only in Finnish). There is also the site of Karstula commune at http://www.karstula.fi/ (mostly in Finnish again). Hopefully these are of some help!