Possibly the Guangdong Geological Museum shop will have some good specimens--at a price. One place that I'm not sure has a retail shop, but is reputed to be really interesting if you're into the subject matter is the newish and architecturally avant-garde Dapeng Geology Museum in Shenzhen--it's on the Dapeng peninsula as part of the volcanic geopark. Haven't been there but it's on my to-do list when I next pass that way.