Hi, Ms.Deborah, if your purpose is to visit some wholesalers in Dongguan, your can ask any of them for an invitation letter, then, apply for a business visa to China in advance through Chinese embassy or consulate in Spain which hold the consular jurisdiction of the place where you reside. But if you can not get a letter, you also can apply for a tourist visa instead, providing return ticket and hotel reservation. In addtion, your residence permit in Spain is required when submit your application.