Go to a tools store like Harbor Freight or Sears to buy a tool..

they have tools to take the rounded nuts out. At Sears, Craftsman makes a tool called "Bolt-Out", specifically for your case.

I'd recommend you to buy a set of good tools, like "Craftsman", lifetime warranty, to make DIY easier. With those cheapy ones people are more likely to mess up bolts/parts which makes one's life more miserable...

Save labor cost, not tools.