The Registry of Interpreters for the Deaf (RID) offers several search tools, such as a directory of both individual interpreters and interpreting coordination agencies. RID also has state chapters that can be contacted for local referrals as well.
Many states have state affiliated agencies supporting deaf people that offer information and referrals to interpreters. If your state requires interpreters to be licensed, the licensing entity may also have public search options for finding licensed interpreters.
Subscribe to the NDC Listserv and seek additional service provider referrals from colleagues.