I bought two extra used but in good condition original MB chargers but can't find the link for the purchase.....i do recall staying away from after market chargers.
Personally, I'd find reputable seller on eBay. I see half a dozen 'reliable' sellers on eBay offering for less than $20, some include free shipping.
http://www.ebay.com/sch/i.html?_from...arger&_sacat=0