There are several ways to find a reputable builder.
1. The Yellow Pages
2. The Home Builder's Association
3. The Better Business Bureau
4. Ask at the neighborhood lumber yard (not the big box store)
When you find 2 or more of them ask for firm bids, pictures of past work, references (which you check), a written contract (estimates won't do), and proof of liability and worker's compensation insurance. Understand up front, you will not pay for work that is not completed; you may want to ask a local bank or real estate appraiser to inspect the job frequently and give you a written report (just to keep us all honest).