As I was playing today, a thought came to me that I figured I may as well suggest.
Basically, the idea is that, on the world map (but before actually launching a mission), there could be a bar/dropdown where you could "prioritize" a resource when you start your next mission. The default setting (and the one it would return to after each mission) would be "none," however, you would be able to then select food, water, metal, wood, cloth or ammo from the dropdown, making that resource prioritized for the next mission.
Prioritization would increase the likelihood of you finding the chosen resource by between 15-20%, but the drawback of this would be that it would decrease the likelyhood of finding anything else (including weapons, clothing, fuel, etc.) by a much greater degree, around 40-50%. This would help players who, in an emergency, are in need of a specific resource while also making it harder to find anything else, so a player won't be tempted to abuse the prioritization mechanic to stockpile all resources.
Any book effects would, for balancing purposes, be calculated in BEFORE calculating the increase/decrease in likelihood.
It is meant to be something of an emergency measure. Think of it like this: your survivors are starving and desperately need food, so when they go out on a mission and look through a pile of rubble or a cabinet, they're more likely to ignore the piece of steel rebar in favor of a can of soup. If they know that their walls are in severe disrepair, they may toss aside a box of ammo so they can carry more lumber. That's what the prioritization mechanic would represent: your leader saying, "take as much (insert resource here) as you can find, Nothing else matters right now."