These are acquired by completing the 'puzzle' at certain Guardian Structures (some give other types of blueprint, like weapons or ships), i.e. by powering up all the towers (these rise up out of the ground when you get near, you shoot the glowing blue part with the SRV weapon) and then dropping an Ancient Relic on the X in front of the big building. You then scan the globe that appears after it is active ( a few seconds). Watch out for Guardian attack vehicles (forget the name - it has been a while since I did this) as they appear over the site as you get near and in numbers once the globe activates. Try to bring a ship with point defence on a top hardpoint, as this will shoot down missiles automatically.
A selection of suitable Guardians sites (not vouching for supercruise distances though):
HD 63154 B 3a
Vela Dark Region DL-Y d112 1a
Synuefe NL-N c23-4 B 3