Craigslist isn't a bad way to check listings, however, some are greatly exaggerated and it is wisest to either accept one on a short term lease until you can familiarize yourself with the area or until you find something that suits you and the children hand-in-glove.
The Hill towns also have a new-ish webpage that might help:
Hilltowns Online Newspaper
And the Hampshire Gazette has listings.
What I've personally found is the farther away from Amherst and/or Northampton the bigger and less expensive housing is in the Valley.