Yeah, you can probably get just the shifter- but it depends on what you've got. Mt or Road? SRAM, Shimano, or Campy? Bar end or fancy lever pushy thing? If it's Rapidfire, Shimano's policy is that you need to throw your bike away and buy a new one . I swear the rep told me this with a straight face. He was totally serious. If it's SRAM you may be able to get a two dollar part to repair the shifter. Guess which one I ride now?
If you want to sell the bike you may want to put on a shiny new matched set for "curb appeal". $$$ wise it depends what you've got on the bike and what will be compatible with it. And you may not want to cheap out on a nice bike if you want to sell it.
So what kind of shifters are they, what's the bike, what else is on it? And how many cogs on the back? (big factor)
Lizzy