I stumble on stuff in other people's reading-lists, mostly. I occasionally click on links here on the boards and find something nice, but most of the time, I'm visiting someone's wall (to say thanks for subscribing, and stuff) and there's something interesting-looking in their reading list. Or someone else has thanked them for subscribing, and the image looks interesting, so I click through to find out what that's all about.
Outside of Tapastic, I usually find stuff via Tumblr/Twitter.