Load cell for Capsule filling machine
Digital scales with range 10g and claimed resolution 1mg are available for low cost. There's no reason why you couldn't remove the load cells and substitute your own circuitry. The resolution is not necessarily an indication of how accurate they will be. You can make the circuitry show resolution of 1ppm, but thermal stability and hysteresis will mean you can't get useful accuracy at anywhere near that level. 

I suggest you try to match the maximum full scale measurement to the load cell as the first step. If you only need to go to 1gram full scale then don't try to use a 500g load cell.