In pretty much any PC controlled setting, you don't have to worry about feedstock: the fabbers are connected directly to the dissassemblers (and other waste management systems), so they are always "topped off". This is also how it is in Titan or any sufficiently large habitat. In a smaller hab, or certain Anarchist/Scum/Criminal ones, you'd have to have your own feedstock as it would always be pretty close to empty (I imagine there'd be someone with limited resources hanging around every fabber just waiting for someone to leave some extra in the fabber, so they could drain it making feedstock). But generally speaking Cordia is right; it should just work until the GM decides it doesn't anymore.