A good resource for used, and limited production new parts is
www.slantsix.org. There are often "group buys" going on over there, either make available parts that are not available as one of's, or give a good price break. One of the lastest group buys was an adapter, from Wilcap, to use a GM 200-R4 trans with a slant six. There have been group buys for Offie parts (valve covers, manifolds, etc). I have arranged several group buys, myself (Cloyes double roller timing set, Copper head gaskets, A/C compressor mounts).
We get together and work with manufactures to bring new parts to market. (K-1 connecting rods, Wiseco pistons, special cam grinds from a couple of different companies. This is a thread dicussing possible new products.
http://www.slantsix.org/forum/viewtopic.php?t=50982