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 Upgrade/Replace the Main Drive in a 21.5-inch iMac (Late 2012 - Late 2015) UPDATED - YouTube
IMac's upgrade PCI-e SSD - Apple Community
OWC Internal SSD DIY Kit for Apple iMac 21.5" OWCDIDIM21SSD11