Crossover nodes have been utilized to achieve smooth handoffs for micro mobility management schemes. IEEE 802.11s supports proxy-based frame delivery services for mobile hosts roaming within a wireless mesh network (WMN). Discovering crossover node for mobile hosts in 802.11s WMNs, however, faces several challenges due to intrinsic properties of mesh networks. This paper identifies these challenges and proposes a scheme suitable for crossover node discovery under 802.11s WMNs. This scheme is characterized by source-oriented, MAP-centric, and per-source. It can be performed off-line with the derived results cached for on-line retrievals during handoffs. Simulation results show that the proposed scheme can reduce packet losses due to handoffs, and confirm the necessity of the source-oriented principle.