There are some good threads about where to find steel, including both junkyards and other sources. Keep in mind that removing spikes from a railroad right-of-way is illegal, and spikes have limited value as stock anyway, mostly because of their length.
Two things I've found to be particularly useful are (1) a local steel supplier (a couple of blocks from my son's school) that sells their offcuts ("drops") for cheap and (2) my mechanic, who in exchange for a couple of small items (bottle opener made from a steering column, for example) has kept me in more axles, torsion bars, and springs than I know what to do with.