My recommendation for source material is to record the source document as a source in your source list, and link that to the person/place/website where you got it as a repository in your list of repositories. This is the way the GEDCOM specifications recommends. Most genealogy software at least loosely follows GEDCOM and will keep your sources and your repositories separate for you.
GEDCOM Version 5.5.1 defines a repository in two parts:
A Repository Record: contains the name and address record of the holder of the source document. Source repositories can be formal or informal. "Informal repositories include owner's of an unpublished work or of a rare published source, or a keeper of personal collections. An example would be the owner of a family Bible containing unpublished family genealogical entries."
A Source Repository Citation: is a pointer from the source record to the Repository Record along with all the information and notes to describe how to find the source in the repository (e.g. call number, specific url, etc.)
So you've got:
United States Census, 1910: is your source document
Page 31, Line 10: is your "where within source" description
Ancestry.com: is your repository
Specific url: is your source repository citation
For an informal source:
Smith Family Bible: is your source document
Page 14, paragraph 3: is your "where within source" description
Mabel Smith: is your repository (person you got it from)
Received from Mabel in 1984 at her house, 111 Front St, London
What is extremely useful to do when you know it, is to record the source of the source. If for example, the source itself indicates it was obtained from somewhere else, you should add a source and repository for where it came from.
This is important to do when obtaining informal information, such as an email that states that the birth date supplied was from the person's birth certificate. Or that a specific piece of information from an online family tree was taken from another specified family tree or maybe from an interview of a specific relative or maybe from a document you do not have access to.
By recording sources (and repositories) of sources, and possibly even carrying that through 2 or 3 or more levels (source of the source of the source of the...), you can build up of master list of sources and repositories, some of which you have personally seen (be sure to note when you obtained/saw each one), and others which you will one day attempt to inspect yourself if possible. This would allow you to verify that the secondary source correctly interpreted and presented the record and also allow you to see if there may be other pertinent information in the primary source that the secondary source did not provide.
Followup: (This is in response to Adrian's comment regarding personal holdings and ColeValleyGirl's note that it should be used for unique documents that no one else has.)
The repository that you supply should be where you got the item, but only if that place still has the item and a researcher would still be able to find it there.
If the item has disappeared, or it is a unique item that was passed to you, then you should state yourself "Personal holding of …" as the repository, and indicate (in a source of the source) where you got it from with a note that the original source is no longer available and when it became no longer available.
This will often apply to websites, people who pass away, records that are lost, etc.