I do not think it is anything that can be tightened as it seems to be just a pull valve with an o rig. I have not looked close enough nor done this, but you may be able to take the pull part out, it is seems like just a notch stop with a with like a wire clip as the stopper, and see how it seals. If it is just an o ring, maybe just replace it or add Vaseline to it.
As far as where to get one, sorry, no help their. Worst part is that, at least mine is, is that they are crimped onto the line. I still think though you may be able to remove the lift up part to repair.
DRAIN TANK FIRST
Also remember that if something goes wrong, and you can not get it back in, you will have no way to fill the tank. So look it over first.
These are just my thoughts on this. Do with it what you may. I have not done the above. Just thoughts on what I may do if I had the issue.