iMac 2013 (late) as external monitor for PC I have an iMac 27" with two thunderbolt ports, and a PC with DisplayPort output. In order to use the iMac as a monitor, it appears that a thunderbolt display output at the source is required (https://www.tekrevue.com/tip/ins-outs-imacs-target-display-mode/), and using a DP-mDP cable did not work (Target DisplayMode on the mac using cmd-F2 doesn't respond).

The question is, is there anything I can do on the PC side (a new graphics card, perhaps? or some sort of a converter) to make this happen?
There's nothing you can do on the PC side.

In order to use the iMac as a monitor, it appears that a thunderbolt
display output at the source is required

This is a misunderstanding of how this works.  There's no such thing as &quot;thunderbolt display output.&quot;  Thunderbolt is a multiplexing of a number of signals:

PCIe
USB
DisplayPort
Power (Thunderbolt 3)

If it was just the &quot;display&quot; portion of Thunderbolt, it would work with your mDP to mDP cable.  You need Thunderbolt connections end-to-end.