My suggestion would be to figure out what differences in nut measurement
you need... eg, determine about what the desired wheel nut would be, then
make a sketch with dimensions and thread spec and contact some
wheel nut vendors  to see if they have anything close.

http://www.mr-lugnut.com/
http://www.performancemarket.com/wheelpa.htm

Alternately you could look the wheels over carefully for markings
(if none found, have a tire removed and check the inside for markings).
If you find some, but they shed no clue as to the manufacturer, contact
the tech support dept of a maker of aftermarket wheels and they can
probably tell from the markings who the manufacturer was so you can
contact THAT maker about obtaining the correct wheel nuts.

All that said, I have a similar problem in that I am missing two lug
nuts for a set of aftermarket wire wheels for a 1980 Cadillac; the company
that made the wheels is out of business, and I still haven't found the
lugs I need
(these have an undersized hex head for socket clearance between spokes...
I did once find a set of these lugs, but the seller wouldn't break the
set and
wanted $120.

I have another similar issue with a missing unique trim ring on a set of
aftermarket wire wheels that came on a '79 Jaguar XJS I purchased.
I've been checking eBay for years, looking for that trim ring.

Buyer beware!

...WayneC