A good source for well established, custom home builders is the Building Contractors Association of South West Idaho,
The Buiding Constractors Association of Southwest Idaho, (BCA), A Residential Construction Trade Association Affiliated With The National Association Of Home Builders.
I would look for builders who have been established for 5 or more years, that would screen out any of the people who jumped in a couple years back when the market was hot. There are a number of registered master builders who have demonstarated their comittment to their craft through education. also, Idaho Power has a designation for effeciency, I beleive its called "energy star". Both are something to look for when selecting a builder.
Its real easy to trash production builders such as CBH or Hubble, but they fill a niche in the market. If however, you are looking for higher quality and are willing to pay for it, there are a lot of good custom home builders here.
Good luck with your search!