Where to find this kind of bolt?
That is the pin or dowel that go to a cam lock for furniture. 

Here is website i found  that has some. If you do not it find there you can check in the furniture parts drawers at you local hardware store or search for furniture parts online.

You will need to measure its length, i would think most are going to be classified in the metric system,  mm or cm. 

Keywords: cam lock dowel or pin, Quickfit Style Dowel Pin Cam Lock Fasteners Furniture Connectors