>>1007150First off, if your shifter won't hold tension (and can't be tightened with the knob), sometimes you can bring them back to life by cleaning them out and reassembling.
But if that doesn't work, to replace the shifters you need to take the shifter and look at the braze-on on your frame - if it has a square shape, it's the normal kind and pretty much any downtube shift lever you buy will fit on it. But if it looks like pic related with a point than you'll need to find the much less common 'B-type' Shimano shifter (Shimano 600 levers are the best known example)
