Gold Medal Models offers several sets of photo-etched metal parts for merchant ships on various scales. Here's a link: http://www.goldmm.com/ The parts on these sheets include railings, ladders, life preservers, and some pretty nice shroud-and-ratline assemblies. (On 1/600 scale or smaller, to my eye at least, the latter look pretty good. I have reservations about them on largers scales.)
Take a look at some of the other Gold Medal products. The sets designed for the Titanic (on various scales) have all sorts of amazing detail parts that would work on other vessels of the period (and, in many cases, later). And many of the parts designed for warships are perfectly appropriate for merchantmen as well.
Several other companies make etched metal and resin ship accessories. A good way to start is by browsing the Steel Navy website: www.steelnavy.com
Hope that helps a little. Good luck.
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