How to get it?

You may build it. Or if you don't want to build the app yourself then just pull the right artifact from latest release and extract it to the root of SD-card on your Flipper Zero. Nightly builds are also available and getting build every day based of latest firmwares version. After that you should be able to run it on your Flipper. Also you may use FlipC.org to get app to your flipper.

To pick the right artifact you would need to know what firmware your Flipper Zero device is running:

FAP file name	Features	Base firmware	Compatibility
totp_official-stable_fw.zip	All	Official Stable	Official Stable and any other firmware compatible with it
totp_official-stable_fw_no-badbt.zip	All except of Bluetooth token input automation (BadBT)	Official Stable	Official Stable and any other firmware compatible with it
totp_official-dev_fw.zip	All	Official Dev	Official Dev and any other firmware compatible with it
totp_official-dev_fw_no-badbt.zip	All except of Bluetooth token input automation (BadBT)	Official Dev	Official Dev and any other firmware compatible with it
totp_xtreme_unleashed_fw.zip	All	Xtreme	Xtreme and any other firmware compatible with it (example: Unleashed)
totp_xtreme_unleashed_fw_no-badbt.zip	All except of Bluetooth token input automation (BadBT)	Xtreme	Xtreme and any other firmware compatible with it (example: Unleashed)

If you are not so sure which firmware your Flipper is running try totp_official-stable_fw.zip first and then if it doesn't work for your then try totp_official-dev_fw.zip.

If you would like to make sure that Flipper Authenticator is completely unable to type code via Bluetooth (BadBT) use the FAP file suffixed with no-badbt. These FAP files are build by completely disabling all the code which deals with Bluetooth, so that you can be sure that Flipper Authenticator is physically can not send anything anywhere via Bluetooth.