Hi, unfortunately, I don’t know of a place where you can buy just that part, however you can search for a dead TI 84 Plus CE on Ebay then use that calculator as a donor device.
Thankfully, removing the plastic cover isn’t very hard, the glue holding it on is pretty weak. You just need a suction cup and maybe a spudger (or thin flat head screwdriver).
# Take the suction cup and place it on the screen, try to avoid putting it over the cracks as that may mess it up.
# Then pull up on the suction cup. You don’t need to worry about damaging anything by pulling of the cover so you can be as forceful as you’d like. The cover should come right off.
# You can now take off the other calcualtors plastic cover and swap your calculator’s plastic cover with the donor calculator’s. At this step you have the option to replace the glue with something stronger so dust doesn’t get in the shell and on the LCD. ''Be sure not to get any glue on the LCD,'' that’s been a common mishap to others who have taken off the plastic cover! Carefully inspect the LCD to be sure it’s clean before putting the plastic cover back on.
That’s it! If you would like me to clarify anything, please reply and I’ll help you out. Good luck!