You can easily find a used one on ebay:
http://shop.ebay.com...-All-Categories
The amplifier is located on the left hand side of the fuel tank in the trunk...it's easy to get to. This amp powers the subwoofer only...all other speakers are powered by the built-in amp in the head unit.
I would beware of ebay used electronics, The seller may have had the same problem as you. I would only purchase a refurbished amp if I were you. Sound advice