After 8 months of full-timing, my shower door sweep is still pristine. Not sure why so many folks are having a problem with theirs.
As for the replacement piece, you can find a "stick-on" door sweep at Home Depot in the door section. The "sweep insert" location will vary by store. Some HD's have them over by the shower doors. Others have them in the hardware dept. by the screen door hardware. They're only about $3.