You can buy a bag of 10 of these for $4


They're intended for Dia-Compe/Weinmann rather than Shimano 600, but I checked the base diameter, and they'll fit. They are about 2.5 to 3 mm shorter in terms of top projection than the original 600 EX tops, which had a more conical than domical shape.

Good luck finding hoods for them. They might take the ones made for Campy-clone lever housings, but the Campy clones don't have a separate ferrule in the top (good idea, considering the angst of losing them), so depending on the hood, the ferrule might stretch the cable outlet a bit. The Dia-Compe hoods (Weinmann, Cane Creek, etc) don't really fit.