I know this isn't what you want to hear...but Fulton Armory has all of those parts listed as in stock and available to ship. (Sorry, man...don't shoot the messenger).
FWIW...the spring is the same from the AR-15 to the AR-10 and LR-308 platforms. Only the rods and doors themselves are different, so if you can locate the rod and door at the same place, see if they have AR-15 ejection port door springs available.
Good luck!