You could shear a sheep in May.
It takes ten minutes. She is grateful. The fleece keeps you warm for forty years. When you're done, you bury it, and it becomes soil within three years.
Or, for ethical reasons, you could choose one of the alternatives.
Cotton. Requires 10,000 litres of water per jumper. The Aral Sea is now mostly dust because the Soviet Union diverted its rivers to grow it. But the jumper is "natural."
Polyester. Crude oil, extruded into thread. Sheds 700,000 microplastic fibres per wash. Lasts in landfill until approximately the year 2226. But "vegan."
Acrylic. Petrochemical, manufactured using a solvent the EU has classified as a reproductive hazard. Marketed as "cruelty-free." The cruelty is in the supply chain.
Bamboo. The plant is innocent. The fabric is bamboo viscose, dissolved in carbon disulphide in a Chinese chemical plant whose workers have elevated rates of psychosis. But it has a leaf on the label.
Hemp. Genuinely fine. Most of what is sold as hemp is a polyester blend, sold at four times the price.
Recycled polyester. Still sheds microplastics. Still ends up in landfill. Made from plastic bottles that could have been recycled into more bottles. The fashion industry has been quiet about this.
Vegan leather. Plastic. Reliably. Always.
Lab-grown fibres. Genetically modified bacteria fed on glucose syrup in a steel tank, in a factory powered by natural gas, packaged in plastic, shipped from California. Funded by venture capital. Not yet profitable.
Or you could shear the sheep in May. She'd appreciate it. The jumper would last forty years. The grass would grow back the same.
But of course, the sheep is the unethical option.