Many times you can find complete Six Series guns (minus the frame) on Gunbroker. These are typically sold by FFL's who have been commissioned by law enforcement departments to destroy firearms that have been confiscated/buy backs/etc.. The FFLs must destroy the frame (the firearm) and they are allowed to keep/sell the rest of the parts. I have bought several of these over the years to supplant what I have gotten at gun shows, Numrich, etc.
Things I would start with are the pawl, pawl plunger and spring, transfer bar, hammer spring, hammer dog, hammer dog plunger and spring, trigger spring, cylinder release pivot screw, cylinder release plunger and spring, cylinder latch, cylinder latch plunger and spring.
Finding complete assemblies - trigger, cylinder, even at times the rear half of the frame, will give you all the little bits and pieces that you might (or might not) ever need.
Most important, buy The Ruger Double Action Revolver - A Shop Manual Vol 1 (there is no Vol 2) by Jerry Kuhnhausen. You should be able to find it on Amazon. This book will tell you every thing you will want to know about repairing the Six Series Revolvers.