Page 133 of the new LMC catalog...
http://www.lmctruck.com/icatalog/cb/full.aspx?Page=133
The spring is included with the shift linkage arm (part #32-4830). I had to buy a new arm because mine was completely wore out and the spring, washer, and retaining cotter pin were included.
But I don't think the spring is available seperately. My local Ace Hardware has all the fasteners and springs I could ever want.
Gary