Tinkeric,
I assume you're talking about Shimano down-tube shifters, as those are the only ones I've seen that use bases that are curved to fit the down-tube. The parts you're referring to are called Lever Boss Covers and provide the limit stops for the levers. You can get the part numbers for them here:
http://bike.shimano.com/media/techdo...9830609197.pdf
Installation instructions here:
http://bike.shimano.com/media/techdo...9830605247.pdf
You can buy'em here:
http://www.loosescrews.com/Products/SH-6433810-1.html
Regards,
Bob P.