Inexpensive AC charger for iPod 3rd gen / 4th gen
You should be able to use any USB charger of decent build quality, as long as they follow the USB spec. Emphasis on decent build quality, you get what you pay for sometimes. The Apple charger has the added advantage of working with other Apple plug adapters the MacBooks use, like the travel kit and those longer cords. Plus they look nice ;)

If you're looking for quality-but-thrifty, try finding unused USB ports on other devices, such as the Xbox 360.