Voidgel Orbs can drop from just about any missions on the Zariman tileset; however, you have to run missions with lots of enemy spawns as enemies can drop Voidgel Orbs when killed. You can also get voidgel orbs as a completion award for simply completing the mission. You can run the exterminate or mobile defense missions since they spawn a good amount of enemies in a short amount of time, plus they don’t take very long to finish.
Voidgel Orbs don’t have the lowest drop rates, but they aren’t the highest, so it’s more like an uncommon-slightly rare drop. So you should get at least a few of them on your first few runs of whatever mission will be running to farm for them unless you get very unlucky.