So you still have to find the spots but one thing I learned after a while was that each terrain type(desert/forest/water/etc) has a prep group as opposed to each spot having a prep.

Once I realized that it got simpler. A number of the spots are not far off of routes you would normally take to get around, hunt, and gather preps. So if you just took the minor detours to grab possible preps as you were otherwise going about your business you would develop a good stock over time.