I've seen a few RGA8 necks on eBay for around $200 - $300 USD a while back, none now but some might pop up from time to time. The ones I've seen were from RGA8's that had the bodies damaged, but I'd check up on the marketplace here and maybe eBay from time to time to see if more pop up.
Sorry if I wasn't much help Best of luck to you!
If you keep your eyes open every now and then Kurt from Rondo music will throw a take off neck up for sale on something that came back all janky, I really doubt he'd be very interested in just selling a neck otherwise though (although some member on here got one for kurt in the 50$ ballpark quite some time ago)
maybe if people showed enough interest in just the 8 string neck's we could convince someone like warmouth to start making them
I'd contact ebay seller loudestbass01. Ive purchased 2 replacement 7 string ibanez necks (one kts) from him that came in factory packaging and untouched. I know he's posted an rga8 neck before because it was hard not to buy it…