Get to the last day of Rats, complete it, pick your card them after the reveal press start and Restart game. It'll restart you on Day 3, rinse repeat. Though I hear people tend to get more masks items from this than mods for some reason.
Open eyes are useless if the mind behind them is closed. 3DS FC: 3222-5762-5282
Ukrainian Job, Standard Jewellery heist and Nightclub are best for me. If you get lucky and the dude with the card is near the security door, you can stealth the whole thing bar the escape. It does require that you buy the loot truck asset though.
Best in the World! GT:Future Shock UK PSN:FutureShockUK. Founder and leader of MP3/GTA V Crew: Hounds of Injustice. Currently recruiting.
3-stars Rats, non-Pro is easily the best because you can restart the last part over and over, instead of having to pray to the Payday gods that Ukranian Job shows up.
1st day, put the wrong ingredient in first time, jump off the balcony to the roof and escape when the lab explodes.
2nd day, just go on a rampage to find the intel, if you have C4 you might get it, otherwise if you take too long they burn it so you can just grab it and run.
3rd day, kill everyone on the bus and grab a bag if you can carry/throw it better. Once you pick your card and it's revealed, press start and choose "restart". Then you can play the 3rd day over and over and get lots of cash, XP and cards.
Lego Harry Potter: Years 1-4 Collectibles Guide: http://www.gamefaqs.com/xbox360/960495-lego-harry-potter-years-1-4/faqs/60378
I would really just wait to start farming cards. Our first update is going to add code that prevents you from getting more of 2 of a single mod. That way you are not just getting same stuff over and over.