You don't need a leech that's decent or higher. You can fight any leech no matter how weak. It's suggested to fight more weaker ones if you fight alone. Most people think Qufim Island is the only place to get one. You can get a ruby off any leech. Having "Treasure Hunter" (Thief Job Trait, Lvl. 15) does help with the search. Overall expect to invest some time into fighting to get one. It's totally random and the drop is 1% no matter which kind of leech you are fighting.