Places I've gotten bikes:

. Thrift stores
. Craigslist
. Garage sales
. The dumpster
. The city dump
. Church tag sale
. Pawn shops
. Acquaintences

In short, always be on the look-out. Talk about it enough that those around you know you are interested in vintage bikes without being a bore. You don't find vintage bikes; vintage bikes find you.

When you find a bike and you're wondering if it's any good, look for:

. Lugged frames
. Forged dropouts
. Quality tubing (there's usually a decal saying what it is; look around the C&V forum to get a feel for what's good)
. Bikes with quality components (even if you think you might convert it to fixed)

Avoid:

. Stamped dropouts
. Department store bikes
. Bikes with low-end components (note that Shimano builds both high-end and low-end stuff)
. Just about any bike with "Sport" in the name. The Sport was often the low level/entry level bike in a company's lineup