maxupgrades.com: Single SSD Drives kit for iMac 27 inch 2012 Models with SATA Blade SSD Module / Heatsink/ Power Cable and DATA Cables for [SSD + HDD] with iMac Adhesive LCD Tape
How-To: Now's the right time to swap your old iMac's hard drive for a fast new SSD - 9to5Mac
iFixit gives the new 24" M1 iMac a proper teardown treatment with a 2/10 repairability score: Digital Photography Review