OWCDIYIM21SSD11, OWC Internal SSD DIY Kit, for All Apple 21.5" iMac 2011 Models | EET
Amazon.com: OWC SSD Upgrade Kit for 2011 21.5-inch iMacs, Mercury Electra 250GB 6G SSD, 18" SATA III 6Gbps Data Cable, Installation Tools and iMac Screen Adhesive Tape Set : Electronics
How to: Install an Intel iMac Dual Hard Drive Kit 21.5" (Mid 2011) - YouTube
Internal SSD Cable Upgrade Kit for Apple 21.5" iMac 2011 Models
SSD Upgrades for Apple iMac 27-Inch Late 2013 - 2019
New 2TB SSD in the iMac! Upgrade kit from OWC is awesome! : r/mac