Why Carpet ??
Personally, I will never put carpet in my boat again ! It only gets wet and dirty and creates a great place for mildew to start. Even the so-called indoor/outdoor variety.I used the interlocking plastic squares available at West Marine, and they're easier to clean, do not let moisture accumulate, and will probably last a lot longer overall. However, if you're dead set on carpeting, most any good marine store can provide what you need.