its all random, there's not a single place that drops only good parts, but a good place to start will be to farm team escape (high stakes, most wanted, late departure) with the available tools like the multibot from tdx or with the multihack from berkay,
or go for drags (the next best bet) with the drag bots
all in all just farm team escape, probably get 2-3 ultra parts in a day if you're lucky, get to farm cash too in the meantime and buy misery skill parts with it