If you have an 83, 83+, 84, or 84+ you can do this pretty easily...
From the home screen go to the distribution menu. [2nd] [VARS]
Highlighted right away is  normalcdf(
Now you should see normalcdf( on your screen
Since you don't have an infinity button (like the 89), type in a really large negative number (use -1 E 9) where E is retrieved by hitting [2nd] [,].
Now enter your z value.
Then hit [ENTER]
Let's say my z value was 5.3
Then my percentage would be normalcdf(-1E9,5.3) .
Rounding to 2 decimal places, this would give us 100% or 1.
As z gets larger and larger, we assume that the percentage is 100% or 1.
Hope this makes sense.