Data loot comes from scanning, and you need 25 different types of it for this achievement (so just getting 25 of the same thing won't work).
Ways to get data loot:
1) Lock onto ships and wait for your HUD to update with full details of the target. This will get a bunch of scans related to shields, drives etc...
2) Use a wake scanner and scan the high energy wakes of ships jumping into frameshift. Easiest place to do this is to hang around outside station exits. This gets scans for hyperdrive related things (many of which you'll need for high level engineer recipes)
3) Scan data points on planet surfaces, these are little red buildings with a pole, the pole lifts up as you approach and you can lock on and scan it with your SRV.
4) Some missions will reward data loot, though that's not as common as being awarded manufactured loot items.
For me it was scanning the wakes that I hadn't been doing, once I did that I got the last few unique things I needed pretty quickly.