12speed
so I'm assuming there are 2 chainrings, and 6 cogs
Are your current downtube shifters indexed?**
modern bar end shifter's wont be available for 6 speed indexing, so if you are set on keeping indexing**, remounting your old DT shifters into a pod mount would work.
To do that you will need....

your old DT shifters
the pod itself: http://www.rivbike.com/product-p/sh13.htm
spacers to adapt your DT levers onto it: http://www.rivbike.com/product-p/sh21.htm
cable stops, that mount where your DT levers used to go: http://www.rivbike.com/product-p/cad3.htm
and some new cables and housing
looks like that'll run in the neighborhood of 70$, not bad

if you dont care about indexing, then you can possibly do this cheaper on ebay using an old suntour friction barcon or a used shimano bar end set in friction mode (have to get lucky on price tho, competetive).
still need the cable stops and housing either way.