How-To: Now's the right time to swap your old iMac's hard drive for a fast new SSD - 9to5Mac
OWC DIY Drive Upgrade/Install Kits for Apple iMac Models
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