Stick with name-brand ones from recognized retailers and you should be ok. They do cost a bit more but it's worth the difference IMO. The cheapo no-name ones tend to be poorly designed and made, with less isolation from the AC line and more chance of damaging your device or catching fire.
If you're in the US and have access to one of the major brick-and-mortar electronics stores like Best Buy or Micro Center you can get one there. Also you can go into any cellular store and they will have plenty of accessories including chargers. They might have both OEM and aftermarket ones, I'd go with an OEM as quality will be higher. It can be an OEM for a different phone, but as long as it's USB and has a sufficient current output rating for your device the brand doesn't matter.
