Does the frame have down tube shifter bosses so you need the Shimano-style adapters shown in norwood's posting? If so any LBS will have a box full of leftovers and you should get them cheap.

If the frame had dedicated braze-on lugs that are threaded to take just the barrel adjusters, the adjusting barrels from the Shimano adapters will work or barrel adjusters from many brake sets will also work as they have the same M5 threading. Again, most LBS's will have a bunch of these as surplus.