Apparently these are called scissor clips and there are various types, identified as type A, B, C etc.
It's important you identify the type you need. To do this, use this handy reference
Full Product Page of Scissor Clips
I have a Unibody MacBook Pro early 2011, which is actually the first item on that page. What isn't immediately obvious is that if you click on the image, a new window opens up showing you images of the various scissor clips for the keyboards of the designated Mac products.
You can then double-check the type against the one you're replacing. So in my case, I can now confidently go ahead and order a bunch of type G standard clips.
I had expected there to be a link from that page to the product listing, but if there is, I can't see it. So instead, go to this page and choose your clips according to the type number you've now identified: SEE BELOW