Some can be recycled from unneeded gear. Titans favor plasteel, which can be found by disassembling old equipment. Hunters unspool discarded armor into sapphire wire. Warlocks extract hadronic essence from dismantled fieldweave robes. And any Guardian with a sense for weaponry can disassemble old ordnance into weapon parts.
Other materials need to be scavenged on site, generally in the course of Patrols. The Cosmodrome in Old Russia is rich with spinmetal, a fantastically light and strong composite created by rogue colonies of Golden Age machinery that escaped storage. Solar coil systems on the Moon still generate helium filaments. The baffling, possibly Vex-influenced flora of Venus grow spirit blooms. And the surface of Mars offers deposits of ultra-dense relic iron.
The most powerful Guardian equipment transcends ordinary science, entering the realm of Golden Age secrets and the Traveler's power itself. This wargear demands Ascendant Energy and Ascendant Shards - burning fragments of the universal fundament, earned through mighty acts of heroism.
Look for them in daily Story challenges and Raids.