Put up my ts of the trial: extracts from the trial of Spencer cowper
Chuancy is on google books.
So is Murray, diary of Mary Countess Cowper
So is the trial and the tracts about it, in cobbett’s Complete Collection of state trials
Cowper’s advice to George I in 1714 is reproduced in English Historical Documents 1714-83 eds. D.B. Horn and Mary Ransome (1957), document 47, pp.194-6 which is on Google Books
Google books has: John Lord Campbell, The Lives of the Lord Chancellors (10 volumes, 1857), vol. 5, pp. 218-351, which contains a biography of William Cowper and transcripts of some of his personal papers.
The National Archives has an on-line listing of the Cowper papers at Hertfordshire Record office: http://www.nationalarchives.gov.uk/A2A/records.aspx?cat=046-dep_3&cid=-1&Gsm=2008-06-18#-1