Post in the appropriate forum here based on the car you want and they will be able to point you to shops or individuals who do that kind of programming.
Vehicles are tuned with a fuel curve that has to suit many parameters - the most important one being emissions. The second most important one is warranty repair. Modern tuning (including higher compression ratios these days) often use slightly richer mixtures to reduce NOx emissions and prevent detonation. Some cars can benefit from leaning out the fuel curve a bit but whether or not it will improve MPG is anyone's guess.
MPG is mostly determined by your right foot. Without physically changing the engine's properties, it takes X amount of fuel to make X amount of power, so if you change the power by altering the fuel curve, you will likely end up using more right foot to get the same driving style you had before and negate any MPG increases.
Those modifications to the computer very rarely increase MPG, they are usually part of a performance tune. I have done it several times on my car. I was hacking in to change the computer for different gears so I used a leaner/performance tuning on my car. All it got me was 10 hp and the need for premium fuel
If you are truly seeking MPG, there are many vehicles that will meet or beat the Prius without tuning.