Just use a 'tourists plug adaptor' and it will work fine. I lived in the US ten years ago and plugged my UK purchased Toshiba laptop into the mains using an ordinary travel adaptor that you get in an airport etc and it worked fine. I bought a Macbook Pro when I was out there - which is what I am typing on now and it plugs into the mains in the UK without any problems.
You don't know need a 'convertor' - just an 'adaptor' If you look on the 'white box' in the middle of the power cable, it actually says 100-240V and 50/60Hz. The supplied power cable does the necessary work for you.
So - I am using a US purchased MBP with the original US power adaptor and all I have is a £4.99 travel plug adaptor and it works just fine.