The FAA Regulations Division maintains a webpagewebpage which includes a searchable database of "Legal Interpretations & Chief Counsel's Opinions". It includes the following disclaimer:
Legal Interpretations and the Chief Counsel's opinions are now available at this site. Please note that not all interpretations or Chief Counsel's opinions are available at this time. This database consists of legal interpretations issued from 1990 to the present and will be updated on a regular basis.
The letters can be searched by year and/or keyword.