Compatible with iMac models A1418 and A1419. Available for around $39 to $52 at eBay - mybagpack.

Apple's Fusion Drive was a hybrid storage solution that combined a small, fast solid-state drive (SSD) with a large-capacity mechanical hard disk drive (HDD). While once a standard option for iMacs and Mac Minis, it has been largely phased out in favor of full SSD configurations. Understanding the Fusion Drive

A Fusion Drive is not a single physical part but a logical volume managed by macOS.

Apple discontinued the last Fusion Drive-equipped Mac (the 21.5-inch iMac) in April 2021. You cannot buy a new "Fusion Drive" as a standalone retail product, but you can find replacement parts or create your own:

It aimed to provide the massive storage of an HDD at a lower cost than a large SSD, while still offering faster boot times.

Standard 3.5-inch or 2.5-inch SATA HDDs can be used as the mechanical component.