Depending on the application, there are lots of options for changing over to EFI from carburetor. Your best bet, especially if you're not a professional, is to find a system which is plug-n-play. Trying to find one which is compatible by retrofitting it from another vehicle would not only prove difficult, but would be very frustrating when it comes time to tune the system.
My suggestion is to look at systems such as ones by FAST or Holley (there are others available). The reason for these has to do with self tuning. All you do is install the unit, plumb the fuel, update the fuel pump, apply power, add in the sensors, put in your engine parameters, and the unit will self evaluate and update as needed for optimal performance/fuel economy.
With all this said, you probably won't see a huge improvement in fuel economy. What you will see improvement in is driveability. The engine will run smoother in most every running situation (as long as everything is installed correctly).