Nemi does not farm them himself, he buys the pelts. One of the main fur farmers these pelts are acquired from actually raises them mainly for exotic pets. Ones that are too aggressive, ill, or unable or undesirable to breed anymore are put down and the pelts sold.
However, there are foxes at farms that are put down specifically for their fur. However most of this fur goes to the fur market where it goes to the fashion industry.
Not everyone agrees with fur farming and that is fine, you don't have to. But these animals are not treated cruelly. They are sheltered, fed abundantly, and allowed to breed before being put down humanely.