I don't think SRS passes drop in LD map mobs at all, or at least I have never seen one in all my time here. As others have mentioned, your best bet is to farm red CCD (a dungeon inside cave of conqueror) for SRS passes. Characters that to are mid-Immortal to HG and up should be able to clear all 20 stages of red CCD with basic gear.
You can get SRS passes by farming materials from Icy. Antlers from reindeers + Plate of North sea = purple passes molar tooth from elephants + plate of North sea = green passes snowmans fur + plate of North sea = red passes But I echo the previous advice to just farm from ccd red. If you will farm in icy for the mats, it's better to just concentrate on farming snowmen, to get red passes. You will out-level icy faster than you think and it's better to maximize the passes you farm to those that are most useful.
