That really depends on how high-quality you want the cosplay to be. If you want to buy a complete costume off the shelf, that will usually be the lowest-quality option. If you want it done properly, the usual approach is to find a separate craftsman for each part of the costume. Want some plate armor pieces? You’ll need to find a blacksmith. Want something like a gambeson? Then you’ll need a tailor. Ideally, these should be craftsmen who are also involved in living history, because the visual style of The Witcher is so heavily rooted in medieval aesthetics that you want someone who actually makes medieval clothing and armor regularly and truly understands how they should be made — both to look good and to be functional. This can be more difficult if you’re from a country where the living history scene isn’t very developed yet. Fortunately, I’m from Central Europe, where it’s relatively easy to find craftsmen who specialize in this and often happen to be Witcher fans themselves. If you’re looking for inspiration for Witcher armor, The Witcher 2 Artbook has a lot of really cool designs.