Visualize a normal curve with 88% in the middle, and the remaining 12% divided between the two tails - so there is 6% in each tail.

Now, to find the z score, you look up the value that separates the upper 6% from the lower 94% - in other words, what z-score has 94% of the area under the curve below it?

Then, due to the symmetry of the normal curve, just multiply this z-score by -1 to get the other z-score.