If there's an Interstate warehouse within reasonable distance, go by there and ask them about their "blems". These are batteries that have a cosmetic blemish that they can't sell at full cost. They'll still carry the same warranty.
If not, then go to Costco.
Costco has a high core fee on batteries. My opinion is they're using it as a profit maker from people like you that might not have an old battery to turn in - especially if you buy two batteries. Check around to see if anyone has an old battery that they'd like to get rid of and take it with you. Interstate will charge you for not having a core also, but I remember them being cheaper than Costco.
Bill