Before you change the LCD, the first thing is to check the Contrast and Backlight settings – these can be changed in Edit Options -> LCD. Because different firmwares have different default settings, which may not be optimal for your LCD, tweak these settings to see if they make any difference.
If you do have to change the LCD – it is an off-the-shelf part you can buy from Amazon or similar online stores. You should open the enclosure, remove the LCD from the circuit board, then check the LCD type. It is a widely available 16×2 character LCD, but depending on when you purchased the controller, it may have 16 pins, or 4 pins (the I2C type). You will need to buy one that exactly matches the type. If you are not sure, take a picture of the LCD and attach it here so we can help you check.