God, how terrible the current systems are! E.g. looking for the Polonium publication from 1898 and can't find it easily! Presumably would be in the Comptes rendus de l'Académie des Sciences 1898. There are two per year, and from searches would be tome 127. Finally found at: archive.org/details/ComptesRendusAcademieDesSciences0127/page/n5/mode/2up
A decent manual list of publications from the Comptes rendus de l'Académie des Sciences: comptes-rendus.academie-sciences.fr/page/marie-skodowska-curie_fr/ They group Marie Curie's Polonium paper and Marie Curie's Radium paper together without giving the title of the second one though, but the page is given.