You're looking for a download link for Lotus Notes Client 8.5.3 64-bit. Here are some details and potential sources:
If your organization requires a true 64-bit client to meet modern compliance or OS standards, you must upgrade to: lotus notes client 8.5 3 64 bit download
IBM sold the Lotus Notes/Domino portfolio to in 2019. All downloads are now hosted on the HCL portal. You're looking for a download link for Lotus Notes Client 8
The base 8.5.3 client has known bugs:
was available for version 8.5.3, the corresponding user client remained 32-bit. HCLSoftware Key Availability Details Architecture Limitation lotus notes client 8.5 3 64 bit download