Many libraries contract with vendors to do authority work.
In fact, it is advisable to run authority control at the same time retrospective conversion is done. Libraries should do authority maintenance on their own on a continuing basis and it is also a good idea to re-run authority control with a vendor every five years or so. Several authority vendors are listed below. The list is by no means comprehensive. Other sources to try are the yearly buyer's guide issues of Library Journal and Computers in Libraries .