Is there any way to use a 2011 iMac as a monitor for a non-Thunderbolt PC? With a 2010 iMac I would be able to connect an HDMI to mini DisplayPort cable from a GPU to the iMac and have the iMac act a monitor using Target Display Mode. However, after some research I found that the 2011 iMac cannot do this and its Target Display Mode only works if you are connecting from a Thunderbolt device.

Is there any cable or converter or even a GPU with a Thunderbolt-output that I can use to get around this?
Sorry, no. There are some PCs and motherboards coming out now that support Thunderbolt, but because of how Thunderbolt works, it's not something that can be added on later to something that didn't initially support it.