Source a material that you like online, then take the material and the original cushion to a tailor - they will be able to copy the original cushion and make a brand new one. This way you get exactly what you need and can choose any colour and style you want.
Sometimes if you can't find cushions for a particular model you can find ones that will still fit. If you still can't find any cushions you could always take what could be the cheaper and easier route, making your own. Even if you don't know how someone you know might.