ebay is a decent source. I like the dealer as they sell it by the foot. I was surprised how many variations GW has used over the years. If you need a lot, you may be able to match it up locally and buy in bulk off ebay.
My GW Dealer carries them. if you are trying to match exactly you should bring a sample bc as previous poster said there are a lot of them. If you are replacing the whole boat then you can pick ant style you like.