The Czech Digital Mathematics Library, operated by the Matematický ústav Akademie věd České republiky, includes a digitized collection of Bolzano’s mathematical works which can be consulted here.
A resource on Bolzano’s logic is the entry Bolzano’s Logic, by Jan Šebestík, in the Stanford Encyclopedia of Philosophy.
The Bernard Bolzano-Seiten, of the Department of Philosophy at the Kultur- und Gesellschaftswissenschaftlichen Fakultät of the Universität Salzburg, include information about the Internationale Bernard Bolzano-Gesellschaft and its Newsletter, the Bolzano-Winter-Archiv, the book series Beiträge zur Bolzano-Forschung and other useful information.
A resource on Bolzano’s work can be found in the entry Bernard Bolzano, by Edgar Morscher, in the Stanford Encyclopedia of Philosophy.