If you use clear bags alot, it is best to order them online (I get mine at clearbags.com) but for a package here and there, definitely the local craft stores with coupons is the way to go.
I search all year round for these kinds of things, and buy them when I see them or have a coupon.
For example, I buy my cutter refill blades or ATG glue gun refills throughout the year with coupons in preparation for using alot of them for craft shows. (I had 6 ATG refill 2-packs that were purchased at 50% off and am down to 1 pack now).
This has worked well because I often find them out of stock. If I really NEEDED them when out of stock, then I would have to buy them full price somewhere!