Several points:
IMHO, you should always be looking for specials at the large supermarkets and discount stores. In Jewel, they generally hide them on a rack near the meat counter on the back wall of the store. WalMart and Dominicks generally put them in the front of the store between the cashier stands and the end caps. Woodman's has a clearance area in every department and places non-perishables in the dozen carts that are set in front of the store.
As for seasonal Ziplocks, you can get them at the major stores on sale right before Christmas. However, some of the best savings can be had in January and February when the manufacturers duno their seasonal inventory at the various dollar stores. The two where I found a significant quantity last year are Deals (Supervalu) and Dollar Tree. My motto in those places is to buy early and often because once they are gone, they are gone.
I have a boatload of seasonal storage bags when my wife went hog wild a couple of years ago and bought 15-20 packages at about $0.25 a package. (And you wonder why all my storage space in the condo is filled.)