Type of SSD for iMac 21.5" (mid 2011)
The iMac 21.5" Mid 2011 shipped with "500 GB, 1, or 2 TB (7200-rpm SATA) and/or 256 GB (SSD)" Storage and its Hard Drive Interface is "6.0 Gbps Serial ATA (SATA)".  So any normal 6.0 Gbps SATA SSD should work.

As an example, from Crucial, Apple iMac (21.5 and 27-inch, Mid 2011) compatible upgrades