I think there is a published catalogue of The Salisbury Papers at Hatfield House. The Lansdowne Collection, formerly held at the British Museum, is now in the British Library on Euston Road. The BL has an excellent online catalogue but is in the process of digitising some collections, so I’m not sure what’s available. Note if you are searching online it is Lansdowne, not Landsdowne.
I recall using the calendar of salisbury papers, but can't find much on google. Should be on the shelf in any big UK university, definitely in the BL.
The Calendar of the Salisbury Papers at Hatfield House are available at searchable PDFs from Tanner-Ritchie. You can download them and keep them. There are several volumes. Tanner-Ritchie holds sales from time to time. I purchased several titles at $10 Canadian a few years ago. You can also find them at:http://www.british-history.ac.uk/catalogue.aspx?gid=144As far as I know the Lansdowne Manuscripts have never been calendared. I consulted them at the British library on Euston as Marilyn R has pointed out.
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