Thank you very much for this and the guidance..Here is the download page, you can select 19.4. I'll upload the version here so you can check checksum since you only use it for upgrade.
Downloads (juniper.net)
Download file junos-srxsme-19.4R3.11.tgz
Download junos-srxsme-19.4R3.11.tgz fast and securerapidgator.net
Here is the latest
Junos Software Versions - Suggested Releases to Consider and Evaluate (juniper.net)
Download file junos-srxsme-21.4R3-S2.3.tgz
Download junos-srxsme-21.4R3-S2.3.tgz fast and securerapidgator.net
The latest suggested is 21.4 now.