Under railroad tracks, of course!!
Okay, I seem to remember that most of the "big box" home centers have them. Clark's hardware (downtown Tualatin, across from Haggen,) has a big stack in their parking lot which I pass year-in-and-year-out.
Price never seems to vary much: usually right about 10 bucks each.
Of course, (wink, wink,) if you can come up with a source for them PRIOR to the time that the garden and home supply folks get them, your results may vary.