First, clarify which area you're talking about-lateral head and inside head are on opposite sides.
For lateral (outside) head, try pushdowns with elbows flared out wide. Do 1/2 or 2/3 ROM avoiding the top of the motion. Also rope attachment pushdowns.
Also, bench dips with knuckles turned inwards.
Use a one-handed cable extension, start with the right hand on the left hip and move the handle over to the right hip.
Also, one-arm extensions with DB. Start with the DB on the left shoulder for example, and raise to over the right shoulder.