How to find good art frames at thrift stores?

I want a more affordable source of picture frames.  

Some of my paintings are wall worthy now.  It turns out the most expencive element of painting isn't time or supplies, it's framing.   Wowzers!  

I have some pictures (mine and others) hung unframed and they get damaged slightly on the corrners.  So frame seems better than no frame.

At second hand shops, they sell a lot of paintings, prints, and empty frames.  Usually for $10 (vs $120+ for a new one) or less.

So, equipped with the knowledge of the usual sizes I paint, I went shopping.

It turns out a frame is not just a frame. There are so many different types.  How do I know which frames are acceptable for canvas or cradled wood?  Which ones for canvas board? Which ones is it fine to remove the glass?  Which ones are crap?

I seem to like frames made of wood better than plastic or fragile metal.

Other than that, I have no idea what to look for or at.

Help?  How do i start learning about second had frames?