Source localization based on time-difference-of-arrival (TDOA) measurements from spatially separated sensors is an important problem in sensor networks. While extensive research has been performed on algorithm development, limited attention has been paid to sensor geometry design. In this paper, we study the optimal sensor pair geometry for the TDOA based source localization problem. Analytic solutions to the optimal sensor pair geometries, for both static and movable source cases, are derived when there exist no communication constraints. Furthermore, in many applications, sensor platforms such as unmanned aerial vehicles (UAVs) and unmanned ground vehicles (UGVs) are movable, and their movements and the communications between sensors are constrained. The problem becomes how to optimize the trajectories for the moving platforms such that optimal source localization and tracking can be achieved. We extend our work to optimal sensor path planning and cast it as a constrained nonlinear optimization problem. The sequential quadratic programming (SQP) method is adopted for a solution. Computer simulations demonstrate good localization performance.