Mark,
There are various search engines online that can quickly point you in
the right direction.

Using a site called "ADDALL" and your grandfathers full name, Isaac
Meir Weissenberg, brought up 24 listings.

Many are print on demand or ebooks, but my hunch is that you'd like some of the
original publications?

Henry Hollander was showing availability of the following titles,
Warsaw Farlag and Tsentral, publishers.

Kin'Ah Un Ta'Avah, Un Andere Ertsehlungen
Geklibene Shriften. Ershter Band
Reale Kreftn
Shriften
Kasper
Yudishe Zamelbikher

You may also want to check yourself using the other possible spellings you
have.

Henry Hollander, Bookseller, San Francisco, USA.
Web: http://www.hollanderbooks.com/
E-mail: boychik@...
An antiquarian and scholarly bookstore and search service specializing
in Judaica, selling quality used books through on-line catalogues and
at their San Francisco shop.

If you haven't already done so, you may want to check further with
The Jewish Book Center for additional leads.
http://www.yiddishbookcenter.org/

My father had several of his books.
Good luck!

Nomi Waksberg