I have been banging my head against the wall for a while trying to find enough Manufacturing Instructions to upgrade my on-foot gear, and finally found a much more reasonable way to find them than grinding crash sites:
Just check the mission board while disembarked! Set the reward display to materials and skim the list. I frequently find 1-3 missions offering each 1-5 of these, and the missions are generally not too bad to complete. My bottleneck is now power regulators – which can be found the same way albeit in a much reduced quantity.
Fairly often these missions have me going to an abandoned settlement as well, which is great for looting everything not bolted to the floor without consequence.