Shimano does make a 9 speed downtube shifter as well as a 9 speed bar-end shifter.

If you wanted to go to 9 or 10 speed you'd need new shifters, possibly a new rear derailleur, new chain, and you'd need to spread the rear of your frame a bit to squeeze the 9/10 speed wheel in there. Your bike is probably 126mm spacing and a 9/10 speed wheel will be 130mm spacing but you can just spread the frame a little and it will fit.