There are really expensive ones like
this one, but i'm guessing that's out of your price range.
If you want to lower the output by a fixed amount you can use something like
this.
But my recommendation is, if you're handy with electronics, you could build your own. Google for audio attenuators, you can get plans for stepped line level attenuators. Or you could just buy a single resistor and put it inline with the signal.
If you build your own make sure none of the electronics you use are polarized. And use high quality resistors with 5% tolerance or less.