So far as IOM censuses are concerned, I think they are organised as part of the ones for England and Wales. The original records were held by the old PRO, London (now National Archives at Kew), and they would decide who could have access and when. They would produce the original volumes if an entry was not readable on the film.
They also used to allow personal requests to have a transcript of a more recent census entry (for a higher fee) but only for the household members identified - they would omit anyone not mentioned. However because of the 1920 Act it is unlikely that they would continue this service for the 1921 census entries.

Sue