Radio comes on but no sound

I have a 2000 540i, the radio comes and but there is no sound coming through the speakers. I took it to the dealer where I got it from (non bmw dealer) and all they can tell me is that I need a new amp in there. The problem is they say is that it's a older car so the parts are hard to find. Is there any help out there where I can get the part, thanks in advance