There are many different types of seats available. Flat board, padded board, pinned, mounts, aluminum tubing, full seat back, etc. Which style would you like? I would start by punching "Quicksilver seat" (With the quotes around it that forces both words to be in the search), into google and start in images to decide which style you like, then get the part number or retailer from a regular google search.
Can't help on where, since you are in Europe. Hopefully others can better answer your question.
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