thats the slider, they always rust and sieze. If i recall correctly from when i refurbed mine, one slider is shorter than the other?
Dont think you can buy a seperate part, im sure its a whole replacement caliper that you will need, but i may be wrong. A guy on here found a place selling genuine brand new calipers for a fraction of what they are from Toyota. I saved the web link BUT, my hardrive crashed and ive had to replace it so i no longer have the link. I recall a new JSpec front caliper was about £70 from this particular place, which is a fraction of what they cost from Toyota, so a rear would have been somewhat less than that.
If you can find a replacement slider (maybe someone on here has one) you will also need a new seal kit. Nick (import pimp) sells these cheap but there is a few weeks delay RE postage.
Im sure someone on here will be able to advise further