May I direct your attention to this page:
http://bulbapedia.bulbagarden.net/wi...n_Pok.C3.A9mon
In terms of getting one in crystal, I think that it may be more difficult to get one in crystal for some reason. Gold and Silver work for the early locations, it seems...did you try different times of day?
By the way, if you get a Spearow or Aipom, keep hitting it. If not, the tree is a Forest tree and won't give you a Heracross.