I wouldn't suggest you try to find a cheap charger, actually. I have read stories of cheap chargers frying iPhones, so I think it's prudent to not skimp on getting one made by a reputable manufacturer or from Apple directly. Griffin makes good chargers if you want to go aftermarket. Or perhaps eBay auctions for used OEM Apple chargers?
i lost my original charger and bought a new usb charger from a site called inspiretech.com i also need to pick up a car charger from them to but im broke but next paycheck, hope this helps some one out... link