If you are looking for a specific letter at the Jefferson Library, there are several sources to choose from, explained below.
There have been three major publications of Jefferson's correspondence:
The first two editions encompass Jefferson's entire lifespan, but are not as comprehensive as the Boyd edition; however, the Boyd edition is still in progress, and only covers the years 1760-1800.
Many of Jefferson's writings have also been made available online.
Information Files (link to Finding Aid): if the letter you're looking for is a particularly important one, there may be a transcription in the Information Files
For additional assistance, contact the Research Librarian.