Would look for an ARP vendor and tell them the thread size nut you want and what it is for. They have just about every nut you could imagine.
The above link should take you to the ARP contact page they have an 800 number and should be able to tell you if they make the right rod nut and where or who to call to get them if they will not sell direct.
Let us know how it comes out on sourcing the nuts.
Oh and if you have to buy a bunch of them I would do it and offer the extras up for sale being no one has them….