It sounds like you have barcons. You may find that it will be cheaper to convert them to thumbshifters rather than buying RapidFire shifters. Check out
Paul Components thumbies.
Speedgoat has them for $48. Otherwise you will need either the SL-R440 (~$60) or SL-R660 (~$100) shifters. And of course you will need brake levers. You can't use brake levers designed for linear-pull or V-brakes because they pull too much cable. You will need something like the Shimano BL-R440 or Avid FR5 (
~$30 from Pricepoint). If you're really looking to go cheap, you might want to check
these out. Unless you can find a 26.0mm flat straightbar, you'll also have to replace your stem.