Guernsey, Alderney and Sark are included in the decennial (1841 onwards) Census for England and Wales and can be found online, see the GENUKI ENG+WAL Census page.
Sark is included on the Guernsey films of 1841 - 1891 (except 1861, see below). 1841,1851,1881 and 1891 have been indexed and there are copies at The Priaulx Library and in the custody of The Family History Section of La Société Guernesiaise. Both undertake research.
In 1861 Sark was enumerated with Jersey and the Société Jersiaise have copies of this film, as of yet unindexed.