That's one of the hardest to get materials in the game. The primary sources where you can get them is by scanning wakes and some mission rewards. In both cases it can take an eternity to get 18 of them.
Of course the standard go-to solution for how to get rare materials is to go to a material trader. However, being a grade-5 material, you'll have to trade other grade-5 materials at an exchange rate of 6-to-1. In other words, you need 108 eg. Modified Embedded Firmwares in order to get 18 Datamined Wake Exceptions.
I don't know which is faster overall, just scan wakes until you get 18 DWE's, or go farm the Jameson crash site for 108 other grade-5 encoded materials and then go to a trader. Either way, it's going to take some work (unless your inventory is already full of grade-5 encoded materials).
(Perhaps the only thing that rivals it in my experience is getting 16 meta-alloys. Can't even trade for those. Although currently you can get plenty of them from the destroyed Titan remains, which ought to make it much easier. But before the Titans it was quite hard.)