TJ Maxx or Marshall's if they have those where you live! Department fashion stores always have last last last season super high end designer jeans under $20.
My wife finds her's at Ross or other similar outlet discount stores. Sometimes they are a bit hit and miss on what they have, but just keep checking back.
American eagle has some too! That's wear i buy all my jeans:) The stretchy ones are really comfy.
Kmart. Selena Gomez Dream Out Loud collection! I wear them, they are cheap, and comfortable!!!
Aeropostle has good sales for jeans right now http://funadvice.com/r/bun380chvon
If you have a mall near you, you are bound to find those type of jeans.
jc pennies. they have changed everything to half off for ever
Forever 21 is pretty cheap or Target.