Using only the brake lever, separated from an integrated shifter/brake
You can use the brake lever without the shift portion just by disconnecting the shift cable. Then, you can attach a bar end shifter or a downtube shifter or similar to shift instead. 

You won't be able to replace just the shifter portion of the brifter (without probably canabalizing another brifter anyway and actually manage to put the old one together). 

You can also buy a 7 speed or so brifter from Microshift still which is Shimano compatible, or the Shimano Tourney 7 speed brifter.