WalMart, Target, RadioShack, Best Buy, and your local Apple store are sure to carry them, and they usually retail for around $20.00. But, you can usually find off-brand 30-pin charger cords for much cheaper at smaller local electronics stores. I got a pink one (off-brand) at a local record store for 6 bucks, and it works just fine. :)
Beware off brands because they have a tendency to break internally! Trust me I have plenty of experience on the topic. I recently got a pack of ten on eBay for four dollars but I guarantee that they will all break within the year or next year.