Yes, I know exactly what you mean.
They are carried by Prime-Line, but that doesn't make them easy to find. A Canadian shop, www.allglassparts.com
, sells something similar for $19 a pair -- I can't tell if they will fit though.
But there's another problem -- in order to replace them the window frame has to be taken apart. The zinc corner pieces (the dull L-shaped inserts) are pinned in place by the dimples in the frame and are a devil to get out without breaking them. Part of the problem is the pinning but another substantial part is they get corroded in place as well.
I think that by far the easiest thing to do is to fabricate pins to replace the broken-off ones. I'll be doing that in about two weeks -- I'll let you know how they turn out.