Earlier, Paul Remde wrote:

...All the US suppliers I've contacted
can supply metric nuts with the right
inner diameter and thread pitch, but
they have all been 8 to 10 mm thick...

My two suggestions would be:

1. Try Mr. Metric in San Jose, CA:

http://www.mrmetric.com

2. Make a drawing of what you need and shop it out
on mfgquote.com. They'd be very easy to make by cutting
down the full-size nuts, and not too hard to make from
raw hex bar.

Bob K.