In space there are few resources. While technologies have been developed to allow warpcraft the ability to garner such resources while not in transit, almost always they are used on concentrated resource areas, such as debris fields, asteroids, novas and the like. Even in dense nebulae the rarity of matter makes resource gathering through the infinite voids of space a bad proposition. Warpships can also land on the planetary/lunar surface to obtain materials in that manner, but anywhere from 0% to 95% of the resources thus gathered may end up being expended as fuel in the effort to get back into orbit. Thus, while populated planets generally have production facilities, military commanders are quick to see the importance of the asteroid belts flung throughout the galaxy.
With the advent of chemicycling, the only resources a fleet will generally ever need are:
From these seven elements can be constructed the wide array of plastics needed for all the needs a fleet may need.
Generally, all of them are available in sufficient quantities from any Mars-sized planet or larger. Asteroids and novas also contain scant traces of these materials, but in varying ratios. The debris of other ships is by far the best way of quickly obtaining the necessary material, though with the development of certain weapons, battles may not leave any debris.