The EW equipment rules are on pages 310-311 of TacOps.
Let me do some digging.
EDIT: Huh. A lot of their light unit EW capabilities were actually in their vehicles, not their 'Mechs. Here's what I found. Also, note that electronic warfare capabilities largely went the way of the dodo in the Succession Wars, so whatever I dig up is likely to be Star League.
Active probe:
MON-66/-66b Mongoose, HSR-200-Db Hussar, OTT-J7b Ostscout, HER-1Sb Hermes
ECM:
SPR-4F Spector, HSR-200-Db Hussar, FFL-3SLE Firefly
TAG:
SL-1G/-1H Sling