Unless you are a realy unusual shape you should be able to get a 3mm suit off the shelf, the thinner material is much more flexable and will stretch to fit most shapes.
My wife is a very strange shape, and needs Made To Measure (MTM) everything except her 3mm wetsuit, she just bought one off the peg at Go-Dive (spent most of the morning trying them all on) and then shortened the arms herself.
If you do still want to go for a MTM suit then a 5mm will give you more flexability. I dive without a suit in water above 30 degrees, and in a 5mm one piece wetsuit in water temps ranging from 30 degrees to 18 degrees, then I dive a drysuit.
As to where to get a MTM suit, I like O'Three. But then I prefer to have exactly what I want or go without until I can afford it. If it's for limited use you'll probably be able to compramise and get cheaper.
now I see, try O'Three they'll make some changes to an off the peg suit as part of the service, I know someone who they said they'd alter an off the peg drysuit for and when they picked it up it was a MTM suit at an off the peg price.