Is there any source for the custom to place notes in to the Western Wall? A very prevalent practice in Israel.
And even if there is no source, is it a good idea?
Taamei Haminhagim (p. 270, footnote) records a story told by the Minchas Elazar of Munkatch, in which the Ohr Hachaim (R. Chaim ibn Attar, 1696-1743) gave such a note to someone to put into the Wall. So it goes back at least that far.