Spinmetal is found by collecting the Spinmetal Leave resource nodes in the Cosmodrome as well as opening chests while in the Cosmodrome. Events in the Cosmodrome may also reward some Spinemetal.
The fastest way to get your hands on Spinmetal is to drive around the Cosmodrome with a ghost shell equipped for resource and chest collecting. The Wombo Detector, for example, will locate resources and cache spawns within a 50-meter range.
Beyond Light updated how Ghost Shells function. They can now be moded to customize your needs. There are mods for PvE, Gambit, Crucible, experience gain and glimmer gains. Masterwork your ghost shell with enhancement cores to get the full utility of a 10 slot ghost shell.
Fastest Way to Get Spinmetal
This is what I’d do. Masterwork your ghost and mod it for finding chests and resources. Do a loop around the Cosmodrome, go to Orbit and repeat. Taking time to defeat any world events that spawn while you’re searching.
Patrols in the Cosmodrome also reward Spinmetal. The fastest patrols are the kill combatant patrols. So… grab a patrol, kill enemies, maybe defeat a world event, circle the Cosmodrome on your Sparrow grabbing resources and chests; rinse and repeat.
This method works with any resource in Destiny 2. Happy hunting!