Quest rewards and coin purses. Neither provides a lot, but over time they add up fast. In beta I got to level 47 and had something like 150g.
You can buy the expansion scrolls on the AH, yes. They're a marketplace only item (300 credits), but apparently people are flipping them pretty regularly. Can buy them for 4-5g. I feel like that's a pretty crazy price, personally, since you're essentially valuing 60 credits as 1 gold. Or, another way, 1g as $0.35. That seems like way undervaluing credits to me.
Edit: Cinge, what craft are you doing? I'm doing alchemy and I feel like it's got to be the most painful to level up. I'm at 2104 skill right now but it's been grueling. Then I see all these other people in the guild with 10k+ skills already and I'm like wtf?