Question: where do saryn prime parts drop?
Answer: The Saryn Prime Systems blueprint is a common drop from the Neo S13 and Axi S5 relics. So if you're trying to also farm for the chassis, then your best bet is Belenus. Alternatively, you've got an equal chance to farm for it from Axi S5 relics which, once again, can be farmed from Marduk in the Void.