After twelve years the prongs on the top rack began to break and rust.

I ordered a replacement from partselect.com and received it in just three days. The only difficult part was deciding how to remove the white plastic end caps on the sliding rails. Finally, I took a small screwdrive and pried out the crimps in the metal, pulled the old caps out, inserted the new ones, and squeezed the metal together again using needle-nosed pliers. Nothing to it!
Note: The configuration of the new rack isn't exactly like the original (size and placement of the prongs differ somewhat) but it fits perfectly and works great.