Or as another suggestion ask at you local computer shop. He might have an old board lying about heading for the bin and for a couple of quid might let you nick the socket.
If you have the bit thats broken off you could try glueing it but I'm an aeromodeller and have many different types of glues and none of them would glue this type of plastic well enough that I would trust it.
can't be replaced. But like you, I can't come up with a vendor.
There are I believe, some heat-sink/fan combinations that fit directly to the motherboard, so that may be worth investigation if all else fails. Or, simply design your own clip to utilize the remaining retaining clips.
I broke something off one of my PCs a few years ago, can't recall what it was but went into my small local PC shop, spoke to the owner and asked if he had an old PC lying about I could get that bit off for a few quid. Got what I needed for a couple of quid, it was no skin off his nose as there was no way I was going to buy a new PC just for that and it was going in the bin anyway.
In the picture it is hard to see, as it cleanly snapped off, giving it that appearance, the 2mm that has snapped off is the difference for the heatsink just pulling straight off as you pull the lever over.
I think ill try PCA towers and speak to Peter.
P.S I know i could buy a cheap mobo for £20 but i am going to give away this old computer to my friends sister who has no money, so trying to limit costings