Follow the MLA format template when citing advertisements in a personal collection. Start by providing the title of the advertisement in the Title of Source element or, if there is no title, a description of the advertisement. If you know the approximate date of the advertisement, you can provide it in brackets preceded by “circa” in the Publication Date element. Finally, use the optional-element slot at the end of the entry to indicate that the advertisement came from your personal collection:

Advertisement for a Royal typewriter. [Circa 1950]. Author’s personal collection.

If you do not know the date, simply omit the Publication Date element: 

Advertisement for a Royal typewriter. Author’s personal collection.

See also our post on citing a photograph in a personal collection and our post on citing advertisements.