One of the obstacles to making medieval crossbows is locating the parts you can't make on your own. I thought I would pass on a few hints. Though every craftsman longs to try his hand on exotic woods, medieval bows were generally made with whatever hardwood the craftsman had available. I usually work in cherry, American walnut, red oak, and maple. I once made a bow out of jacaranda... a type of rosewood. Never again: it was like trying to shape a brick. Beautiful, but awful to work.
As for those special parts, I buy steel prods/laths from two sources. Alchem Inc. of Euclid Ohio (USA) makes good-quality steel prods from 50 to 100 kg. of draw. They also sell bow-irons and a decent pre-made trigger/tickler and Delrin roller-locks. Darkwood Armory, in Hattiesburg Mississippi (USA) makes steel prods of about 75 kg. draw, as well as 'gafa' or 'goatsfoot' levers to order at a very reasonable price. Both of these sources can be found with a simple web search.
I have discovered that the best horn (antler) for roller-nuts is either moosehorn (American moosehorn... I don't know about the European variety) or Axis stag. American elk horn is MUCH too pithy to make good crossbow parts. I get my moose-horn as 'stems' from Moscow Hide and Fur, of Moscow Idaho. They too have a web presence.
For bone placques, the best material I have found to date is cow-bone, but I would dearly love to find a better source. Doesn anybody out there have ideas?