They are sold as complete hub bearings. Depending on the ABS system it has 3-4 sensors. The ABS light is not on because the computer does not know there is a problem it thinks a wheel is locked up. You need to have some testing done to find out which one is bad. The testing could be as simple as monitoring ABS data with a good quality scan tool to look foe the one that reads 0 before the rest. A good scan tool will sample fast enough to show the problem. A cheap scan tool may or may not be fast enough.