Have you considered buying online? I don't know how much shipping costs are, however. I've seen sites where if you're in auto-enrollment there are discounts though, which may make up for the shipping fees. Petfooddirect.com estimated shipping to me at 9.95 for the 18 lb bag, but they offer an automatic 5% coupon. I don't know your zipcode, but you could get on their website and try it out. The actual bags of food on their site were comparable to what my petstore charge (that's the brand we use for our kitties).
Good luck! Our petstore ran out of it for the kitties at one point too, I was in a similar panic!