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 a 2.5-inch SSD in the SSD Drive Bay of a 21.5-inch iMac (Mid 2011) iMac12,1 - YouTube