I mostly build wingy thingies. But I was trying to restart a long-ignored ship build. The Pit-Road 1/700 USS Callaghan, to be built as the USS Johnston. I think I started this thing in 1999. Made some improvements, stared some assembly and clipped a few parts off of the sprues. Now that I’m back to it, 
, I see that almost all of it is still there. But I have lost one of the funnels.
My understanding is that Pitt Road Skyway has long since ceased to exist, so I doubt that I can get spares from them. And 15-20+ for a new kit for one small part seems a bit foolish.
I looked online but while folks are selling 1/700 funnels none are for Fletchers.
Are there any sources that I haven’t checked? Is there a simple way to fashion a reasonable replacement/ Anyone have a spare funnel for sale? 
Not sure how closely the two funnels resemble each other, but you might try resin casting a replacement using the one you still have to make a mold. Most hobby outlets these days have resin casting kits available.
Only other thing I can think of is to carve a new one from wood if you can find some drawings to work from. If no drawings, then the TLAR method…
Also, you might try jumping over to Ships of Scale (shipsofscale.com) to try your question there. Mostly wooden ship builders, but some plastic kit modelers, and a bunch of scratch-builders. All very friendly. You might get some goodness from them.
The funnels are different, the rear one is slightly lower/shorter.
Depending on individual ship and modernisation/date there could be “things”
attached to the funnel.
Scratch building a funnel:
David Doyle pictorial nr 45 (book) + materials and work
two photos in that book
or Google search for pictures/references
or buy a new kit for roughly the same money
What class is Callahan? Look on evil-bay. Might have a used kit of it or the same class for cheap. Also try 1/700 destroyer parts. Might find someone selling just sprues.
Based on a quick review of the instructions, a funnel should be fairly easy to scratch build–maybe an hour or two of work. The basic shape is cylindrical. That can be carved and sanded from a correctly sized brick of plastic. The rim around the funnel is a piece of thin strip stock or stretched sprue. The base is another brick of plastic sanded to shape. If you lost the top of the funnel, that is rectangle with some stretched sprue added in a grid pattern.
A scratch built replacement will probably not look as good as the original, but it should look fine in a display case. You have the other funnel as a guide which will make the job much easier.
I have recently been through a similar problem. Casting supplies will cost more than a new kit and there is a learning curve. However, it is a good skill to have. One sprue, if you can find it, often costs nearly as much as the kit once shipping is added.
I will second Damraska. A few scraps of Evergreen styrene rod and flatbar will make one. In a pinch, some scrap sprue can also be used, but will take a bit more work.
Hey!
Tankerbuilder here from Fine Scale and other places. Do you have any larger plane or car kits! The larger sprue can be used to make a new funnel using the existing one as a pattern! They were Both the same heighth. As far as the side vents you can use.010strip with a .020frame around it!