“See Life of Pym; in Lardner Cabinet Cyclopaedia, Vol. xci., p. 213.”
Fuller cites Forster's Life of Strafford (and Life of Pym) in Lardner's Cabinet Cyclopaedia three times as footnote sources for historical detail. Fuller both quotes from it (the 'soul's dark cottage' passage, attributed to 'a then living poet' = Edmund Waller, 1686, but Fuller quotes via Forster's Life) and uses it for historical details about Strafford and Pym.