ClassicCar

You are looking for scarce items for a scarce year of Hudson! Fortunately the body is similar to the more-popular '37's so certain items might interchange.

Have you tried Cecil Nuxoll out on the west coast, for repro horns? http://trumpethorns.blogspot.com/ (The Hudson-specific bracket, of course, might not be so easy to find, though I think Bill Albright once made repro's for '37's and that may be the same as '38 -- you might contact Jason at Vintage Coach, about that.)
http://www.albrightcars.com/

The fender skirts have been reproed over the years by various people; a good "place holder" until the real thing comes along. Try vendors #7 and 34 (Gleydura and Agape Auto) on my repro list at http://www.detailedconcepts.net/Repro_Table.pdf

The fender-mount tire covers might be adaptable from some more popular make -- possibly someone will jump in here and say something about that.

I assume you've contacted all the Hudson parts vendors who advertise in the WTN, with your needs.

One last possibility would be to contact the person who maintains the registry for all known surviving 1938's, and start contacting fellow owners. (You can also look through your Roster if you're a member of H-E-T, to find other '38 owners.) You never know when someone may have accumulated a "spare" or two that he'd be willing to part with, if the price is right.

Anthony, I assume you are a member of (or at least are in contact with) the Hudson - AMC Car Club Of Australia? Also, if you're in the H-E-T (worldwide) Club you could put an ad in the WTN -- the newsletter is probably read by more people than those who browse the website here.