Depends on the type you need. Note that as your level increases it becomes harder to get certain items from mobs.
SBL (Low qual) - Ratmen , Little Lizardmen, Slime, Slow slime, Ice vivi, Tomb fox.
SBL (Med qual) - Harkans, Ice vivis, Tomb fox, Goblins
SBL (High qual) - Orc Hunter, Goblin, Harkan
*Taken from the FF wiki*
http://fiestafan.com/wiki/Summoned_Beast_Leather