Vertical Take-Off and Landing (VTOLs) vehicles are used to move passengers between skyports in urban air mobility. Safety landing sites (SLSs) cover the trajectory of VTOLs for emergency landings. We study the optimal placement of SLSs in the air transportation network under budget on SLS installation. We propose two models based on the k-splittable and the un-splittable multi-commodity flow problems. We develop edge and path formulations for each model of the problem. The two edge formulations are solved by a branch-and-bound algorithm. We propose a branch-and-price approach to solve path formulations. We perform numerical experiments on a set of automatically generated instances.