You should check out: www.ClassicBuicks.com and/or www.RockAuto.com Classic has seemingly anything for restoring/repairing 60's and 70's Buicks, and more. As far as Rockauto, they seem to have reasonable prices, a site online that allows you to search that far back in time, and good service. Bought a couple things from them, including lower ball joints and a motor mount for a 66 Rivi.
