Hello, My friends and I will be traveling from Seattle (United States) to Shenzhen,

then from Shenzhen to Xiamen. Since I know Xiamen is one of the city that can enjoy the 72 hour free transit, my question is do my friends and I need a visa for the stop in Shenzhen? (which has a layover of 2 hours and 10 minutes). And if so where can we get such a visa?