There was a guy who used to advertise in the back of GW mag. I bought one a few years ago. He calls it "the right one" as he thinks all current tape measures are more suited for left handed people. Try tyring "the right one" into Google and see if you can find it. If not. post again and I'll have a dig.
Peter Helmsley at the Toolpost.co.uk certainly used to sell them, that's where I got mine from.
It's a very useful piece of kit, even if you are right-handed. It saves getting 68 and 89 mixed up, for example.
It boils down to the difference in using the tape when measuring and when marking. When measuring the length of something it is natural to hold the tape in one's dominant hand, but when marking it is natural to hold the pencil in one's dominant hand.
Going personally ambidextrous would be easiest. But on the other hand (ha!) if you want an ambi tape measure, the cheapest option would be to use a Sharpie marker and put on left-handed numbers. Beware though, I have copyrighted this idea and get a royalty for every number you apply. Any form of legal currency will be fine...