Just keep playing. play in tourneys and try to win a credit. 10 credit = 1000 clintz.

if you're lazy and have some extra cash, go buy some credits in 'the shop'

Also tou can trade card with players who are interested in cards that you own.

Or you could save your credits and buy the 50 credit full deck pack, which is the best value of all the packs. I've gotten more rares from those packs than I have the 3 card packs. Granted the odds are better since there's 8 as opposed to 3. However you can always sell your unwanted cards (as long as you have 8 left, for clintz).