R. Yizchok Blazer zt”l (Peterburger) published the Sefer “Ohr
Yisrael”, a collection of writings and letters from R. Yisroel
Salanter. There is no mention at all, of the 13 principals.
The first one to quote the 13 principals of R.Y.S. is
the Mekor Baruch (B. Epstein). He brings no source, from
where he got it.
It is possible , the Mekor Baruch confused the author of the
13 principals with the Sefer Cheshbon Hanefesh. This
Sefer has the 13 Principals. It was republished (1845) with the
encouragement and involvement of R. Y. Salanter.