I'm getting ideas from various forums including Android Central, XDA, RootzWiki but also from different blogs and even by browsing the market occasionally.
I look all over the place for apps, Google play, Android central, and other random Google searches. Whenever I see a cool looking app, I do my research xD. Good thing you can always uninstall them.
Android Central is my recent source due to the new editor but previously I would prolly go through appbrain (which kinda sucked) or the top display on play.google.com
Majority, I find through my Android circle on g+, and then look at the full review at AC and other sites. Few from word of mouth, and fewest by looking at the Play store.
Not to butter you guys up, but I honestly try out most of your "apps of the week". Aside from that it's often recommendations from others on the forums (here or XDA) or friends.