So you need the retainer and not the keepers? The keepers are available on Rockauto but the retainers????? They have to be somewhere, I would look for the old ones, they don't just evaporate.
318cid V8 : Engine : Valve Spring Retainer Keeper
SEALED POWER Part # VK66 {#VK66R}
2 groove; Hardened
SEALED POWER Part # VK138 {#VK138A, VK138R}
w/.371" stem dia.; 1 groove; Square cut key groove; Hardened
w/.371" stem dia.; 1 groove; Square cut key groove0
SEALED POWER Part # VK174 {#VK174R}
4 groove; Hardened
http://www.ebay.com/itm/Edelbrock-9720-Valve-Spring-Retainers-/200721388954?_trksid=p2054897.l4275