James Boswell doesn't have as great a webpresence as one would think, his popularity taken into consideration. Still, there are some links out there which can help in your research into both Boswell and the other persons and events mentioned in his journals, letters and memos.
www.jamesboswell.com - (a very interesting site containing a lot of information about the writings of and publications about Boswell, Dr. Johnson and Hester Thrale/Piozzi)
LoveToKnow Free Online Encyclopedia - a free online version of the 11th Encyclopedia Brittanica, covering a lot of names and events from Boswells days.
Questia, The Internet's Largest Library - a subscription-based online library, offering more than 400.000 books and journal articles online. Some Boswell-related books and articles are in there as well, including the Boswell: The Ominous Years 1774-1776 volume of the Yale trade editions.