The ignition coil is inside the distributor. If the coil is breaking down then you'll have to replace the whole distributor. They'll usually work ok when the engine is cold but start to misfire as the engine warms up. Last one I replaced was about $120 aftermarket (Cardone). You can go "bone yard" but who knows how long they'll last. Not an uncommon problem on Nissan 4's and sixes.