Which SATA cable do I need for a SSD installation in an iMac mid-2011?
By SATA 6, I assume you mean SATA 6Gb/s. This is now known as SATA 3. (In fact SATA 3Gb/s, is now known as SATA 2.)  SATA 3 has been around since 2009, and is backwards compatible with SATA 2, so a SATA 3 cable is fine. Also SSDs need both a power AND a data cable to function.

Apparently it is possible to add a second hard drive to this model of Mac (behind the CD drive). You do not state in the question if you merely wish to replace the hard drive you already have or add a second one. This Ifixit guide shows how to add a second drive. From the guide it is clear that this is a MAJOR undertaking. 

If you simply wish to replace the drive you already have, as seen here, then you should have an easier time. However, according to the Ifixit guide (step 7)


  The WD Caviar Blue drive is a non-standard SATA drive. The power connector features 7 pins, rather than the standard 4. The drive also has non-standard firmware. This means you can only use Apple's own hard drives and not any standard after-market SATA drive.


Thus you need to choose a compatible new drive. In this case you can just reuse the cables from the old drive.