isolation between lcd 20*4 lcd and tm4c123
I suspect what you are seeking is not truly isolation but level conversion.  Depending on the number of pins (typically one can use 8-bit or 4-bit data for LCD control) and direction (most of your pins are probably unidirectional), you can search for level converters at any of the semiconductor manufacturers' web sites such as this one for Texas Instruments.  

Alternatively, I believe the part on that board is 5V tolerant, so you might want to read this application report on what that means and how it might apply to your situation.  In brief, 5V tolerant GPIO pins can generally be used as inputs, but an external pullup and open drain operation are likely to be required for output.  It may be possible that you wouldn't need anything but a few resistors to drive the LCD.