Where is the reorder buffer (ROB)?
Somewhere which is not part of the user accessible memory (i.e. not in the main memory nor in the cache -- that would be too slow, one of the main use of OOO is to hide part of the latency of main memory and low level caches).  Probably in register files inside the processor or possibly in the same kind of memory cells used for L1 caches (but not in the L1 caches).  Modern processors have lot of internal state which is not directly accessible.