Hi Ian,
i suggest you look on eBay, that is where we buy zips from. Regarding plastic v metal sliders, they are generally alloy and therefore do not stain fabric. You can buy a lubricant for zips, this is sold as non-staining also which may help stop corrosion, although to be fair those zips are generally not exposed to saltwater in my experience.
The membership association for owners of Dufour Yachts