Certified iMac backup devices

They don't exist. Apple does not certify vendors of external storage, and Apple does not make an external storage device.

You can use just about any external disks for Time Machine backup. It should be approx 3 times the size of your actual storage use, but if you are going to be quickly growing your internal storage usage, then go for 3 times the size of your physical internal storage.

The external device can be a rotating hard disk, an SSD, a Network Attached Storage (NAS) device. Although for Time Machine I personally prefer direct attached storage, over network attached storage, unless this is a desktop system that will not be put to sleep and carried to a new location outside the home.

There are also alternative backup solutions such as Cloud backup vendors, as well as 3rd party backup utilities that use a different format from Time Machine.

When it comes to backup, the best advice I can give is 3, 2, 1

• 3 copies of your data, with the original being the 1st copy.
• 2 backup devices using 2 different backup utilities (protects against a single point of failure, as well as a bug in one of the backup utilities).
• 1 backup is off-site (this protects against natural disasters, fire, and theft). This can be a cloud backup solution, or a network accessible backup device at a family or friends home (preferably a good distance away), or a regular external disk exchange with one stored at at work, in a safety deposit box, at a not that close family (or friends) house.