STOwiki has some guides to help. One of the capital ships has a small chance to spawn in each of the Aiding the Deferi daily missions - they're not always there.
Easiest to find, if it's there, it'll circle the outpost following the satellite line. Loop around the satellites clockwise and you'll find it quickly if it's present.
Istapp can be hard to find, it spawns far out from the mission area and its path never brings it near any landmarks you can use to track it, so even if it is present, it can be impossible to track down. I suggest ignoring the mission and heading straight to its starting point (northeast point of the square). You should arrive before it gets out of range, but follow the north side of its path to make sure before giving up and going back to the mission.
Ishtapp seems to always appear on the scanners tho it's a way out of the mission area- EASY to find.
Desna usually appears deep inside the asteroid field or running the route around the satellites. I find her fighter cover almost all the time even tho the cap ship doesn't spawn.
Snosk is a mother to find as it's an extremely rare spawn, but when it does spawn it'll be almost within firing range when you spawn into the map.