amp repair?

try ampmedics.net or you could go to phoenix gold directly

If you're OK with opening the amp up yourself, that problem can often be fixed by spraying a cleaner solution into the gain control and rotating it back and forth. This assumes of course that the control is an "open" style and not sealed. If the control is sealed, it'll need to be replaced.

Radioshack - Products

Unsolder it & remove it, then measure the resistance across the outer terminals. That's the resistance value. If the amp is 2 channel then the control will have 2 identical 3 terminal sections. Then go to Mouser electronics or DigiKey and search their list of "potentiometers". You'll need to find something with the same physical size and terminals.

Since it's a gain control, you will want an "audio taper" pot.

search example -

dual audio potentiometer | Mouser Electronics, Inc.

it would be a good idea to replace the caps while you have it apart ...