The easiest option would be friction thumb shifters, they don't care how many speeds you have. Sunrace makes 6-speed indexed thumb shifters that are ok, but not great. If you can spend a little more, I'd get a new chain and freewheel and upgrade to 7 speed, Shimano 7-speed rapid-fire shifters can work pretty well.
The spacing between 6-speed and 7-speed is different. I think this technically means a 6 speed freewheel and 7 speed shifter shouldn't work, though I think I remember getting them to work once. Maybe I'm thinking of 5 speed shifter with 6 speed freewheel though.
Last edited by Arrowana; 06-23-12 at 10:35 AM.