Realizing we don't all approach this the same way, examples from my files follow.
In 2011, my cousin Bill sends me digital images of Great Uncle Joe's estate file. He tells me these copies were obtained as "Case 4301" from the Columbiana County, Ohio, Probate Court in 1988. The file contains Joe's Will, dated 1823, proved 1824, calling out his wife and seven children. I use this document as a source for the relationship between Joe, his widow and the seven children, citing this as >> Will of Joe XXXX (1823); digital images, supplied by Bill XXXX, 2011, citing "copies obtain as Case 4301, Columbiana County, Ohio, Probate Court," 1988; will dated XX-XXX-1823, proved XX-XXX-1824.
In 2012, my cousin Susan sends me a newpaper obituary about our Great Aunt Nellie. Susan doesn't know the date or name of the newspaper. She explains it came from a box of things left to her by her mother. The newspaper article calls out Nellie's surviving children. I make use of this information in various ways, citing it generally as >> Obituary of Nellie XXXX (deceased "last Thursday"), newpaper unknown; date unknown; digital image, supplied by Susan XXXX, 2012, citing materials inherited from her mother.