Sue,
I had to have my drivers side axle seal redone right before Moab, and the mechanics trashed the ABS sensor when they tried to take it off to get the rotor out of the way so they could get to the axle.
The sensors are little magnets that sense the "tone ring" as it spins by on the back of the front rotor. The sensors are made out of platic and are notorious for "welding" themselves to their mounting holes over time. I actually had a new one shipped to The Stoop, and replaced mine right there in the parking lot on that Monday. The sensors are something like $400 so I was NOT happy when the mechanics trashed mine.
I suppose one from a u-pull-it yard would work, but the problem would be getting it off of the vehicle it's attached to without trashing it in the process.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless