Try these guys. They list a 1984 c10 distributor but not clear if with or without vacuum advance.

http://search6969.used-auto-parts.biz/cgi-bin/search.cgi

I just looked at RockAuto and they show the AC Delco unit p/n 88864755 available.

http://www.rockauto.com/en/catalog/chevrolet,1984,c10+pickup,4.1l+250cid+l6,1030903,ignition,distributor,7108