You can rent cars in the centre of Jerez easily or from the airport. You can often get some good deals from the airport companies. I have included 3 car hire companies that are in Jerez.
ATESA: 956313206, www.atesa.es
AUTOGES: 956311900, www.autoges-sl.com
AVIS: 956314 120 www.avis.es
At the airport you can find: EUROPCAR, AVIS, CENTAURO, NATIONAL ALTESA and CROWN