The latest version of Banktivity (8) can be downloaded using the following link:
Banktivity (for use with macOS 10.15-12)

The following links can be used to download older versions of Banktivity/iBank for Mac*:
Banktivity 7 (for use with macOS 10.13-10.15)
Banktivity 6 (for use with macOS 10.12-10.14)
Banktivity 5 (for use with macOS 10.9-10.12)
iBank 5 (for use with OS X 10.8)
iBank 4 (for use with OS X 10.6-10.8)

You can also use the following links to download Banktivity/iBank Manuals:
Banktivity 7 Manual
Banktivity 6 Manual
Banktivity 5 Manual
iBank 4 Manual

*NOTE: Older versions of Banktivity can no longer be purchased.  The only version of Banktivity currently available for sale is Banktivity 8.