Dear Dr Bergstrom,
                                 The source is William Knowler, ed., The Earl 
of Strafforde's letters and  dispatches, with an essay towards his life by 
Sir George Radcliffe. From the  originals in the possession of his great 
grandson. (2 Volumes. London, 1739).  The Strafford papers have been available on 
microfilm for several years and  are much fuller than the extracts published by 
Knowler. I hope that this helps.  With best wishes,
                                                           Christopher 
Thompson