Seriously, contact track frame builder - they will have some. If you are pressed you can use stainless Allen bolts. But track-frame builders will have old stock.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
As above - there are plenty online. I just bought a pair of 30mm stainless steel ones from Velo Heaven. For some reason, knurled end nuts were more expensive than plain hex ones so I went for the latter. Functionally no different. I may put a dot of thread lock in the nuts come final assembly and will certainly stick a tap through and grease the thread with copper grease - just because they never really move between when they're set in position and when you next come to re-spray the frame.