There are plenty of mini displayport adapters and boxes, google to find some, looking at a few pennies though.
iMac (21.5-inch, Mid 2011) and iMac (27-inch, Mid 2011) support Target Display Mode via Thunderbolt to Thunderbolt cable (2 m) when the source is another Thunderbolt-equipped computer.
iMac (27-inch, Mid 2011) computers do not support Target Display Mode via Mini DisplayPort cables.
If you already knew the answey why ask, and that just made no sense for what you wrote -
AFAIK, only the 27" models support TDM, thougth you just said the same two models do yet don't, lol.
I know the 27" does, next to positive, Google for the 21.5, but I don't think so.
There are adapters out there such as the Belkin AV360 (the one I'd go with personally), and others (there are threads, too)
I think you're confused. That was from Apple's support page. The only way I would be able to use the 2011 27" iMac as a PC monitor is if the PC has a videocard with TB. As far as I know, GPU makers aren't talking about going to TB.
I think your confused, check this out: https://forums.macrumors.com/threads/962633/
Maybe I misinterpreted? Its possible to use the iMac as an external display for MiniDP natively (but only up to 720P or 1440P) and with HDMI devices up to I think either or resolution.
Symptoms
When you attempt to connect an iMac (27-inch, Mid 2011) computer to another computer with a Mini DisplayPort to Mini DisplayPort cable, the screen of the target iMac will not become an external display when you press Command-F2 to activate Target Display Mode.
Products Affected
Thunderbolt cable (2 m), iMac (27-inch, Mid 2011), iMac (21.5-inch, Mid 2011)
Resolution
iMac (21.5-inch, Mid 2011) and iMac (27-inch, Mid 2011) support Target Display Mode via Thunderbolt to Thunderbolt cable (2 m) when the source is another Thunderbolt-equipped computer.
iMac (27-inch, Mid 2011) computers do not support Target Display Mode via Mini DisplayPort cables.
As much as I think this sucks, from what I can gather, no manufacturers other than Apple are really seeing any reason to adopt Thunderbolt - why does the average user need a 10Gbps connection for anything?
I mean, look at firewire. It's far faster than USB or ethernet, yet it never really took off and it seems now even Apple is abandoning it.