If you want to EARN credits, you can win tournaments in the 'top scores' tourneys... or you can just enter the 'sweepstakes' tourneys (random draw).
If you want to BUY credits, you can use a credit card, PayPal, gift card etc...
There is no other way...
(well, you do get a small credit allowance each time you change tiers..
- Hack to Amatuer - 50 credits
- Amatuer to Pro - 100 credits
- Pro to Master - 200 credits.)