You should consult the Catalogue Collectif suisse des affiches, or its English-language version, the Swiss Posters Collection, which lists the posters that are conserved in the Bibliothèque as well as other libraries and museums of Switzerland. Unless expressly prohibited by a beneficiary or beneficiaries of the copyright, the notices generally include a thumbnail of the poster. It is also possible to do your search by the name of the graphic designer or collectivity, words in the title, subject, iconographic theme, or call number of the copies

Access to original posters is limited and by appointment only. Originals are only displayed during exhibitions or in cases of serious in-depth research, when they can be viewed under the supervision of the person in charge of the collection. That person is solely authorized to handle the items during a consultation.
Requests for reproductions must be submitted to the person in charge of the collection. There will be a fee according to the reproduction rates in force at the time.
Loan requests must also be addressed to the person in charge of the collection.
Display of original posters is subject to strict conditions in terms of security, lighting, and humidity, which must be validated prior to the opening of the exhibition by the librarian in charge of conservation at the Bibliothèque.

You can request further information online, by telephone, or in person by appointment.