I don't know any cheats, but I would suggest that you figure out what monsters will drop particular virus cores and target them. One of the Ryu books that chronicles the monsters will help with that, it tells the monster's level, where it can be found, and what it drops when you data drain it. You can keep going back to the same level and just data draining the monsters that you know can be holding the cores that you need. It's not a guarantee that you'll get the core you need, but it helped me out a lot since it does get pretty tedius after you beat Macha and Tarvos until you fight Cubia and Cornbeik.