Leaking Kitchen Faucet....Where???

Hello,

I have a faucet in my kitchen almost identical to the one listed below:

Basically, it's leaking. I thought the leak was at the tip of the faucet, and all I need to do would be to replace the screen, washer and nut...but the more I think about it... When I push down on the lever shown in the photo that shuts off the water to the tip of the faucet, the water doesn't drip or leak anymore. This kind of indicates that if I replace the washer "or something" just below this lever, it will stop the leak. First off:

1) I'm not sure if this is indeed the problem. How do I determine the source of the leak?
2) If it is indeed the section just below the lever, how do I take it apart? I see what seems to be a hole for something like an allen wrench, but I don't think it is exactly the same.
3) Can you purchase a screen,washer and nut assembly at a hardware store without buying a whole new faucet? Not even sure if this will stop the leak.

Any advice would be helpful.

Thanks samadams snipped-for-privacy@yahoo.ca