Best place is actually e-bay or swap meets, for non wear items. Pistons, rings and bearings show up on a regular basis. As do intakes (Clifford and Offie). Aussie Speed has several slant intakes. Headers for a 67 or newer "A" body are available from Hooker, Aussie Speed (frank, 66acuda on this site) and even Clifford (only part I buy, and only if in stock). Several sources for cams, both new and regrind.
Take a look at www.slantsix.org for more sources.
Here is a link to the build of my Valiant, just to give you ideas. http://www.slantsix.org/forum/viewtopic.php?t=11218