I think source selection should be done in the executing process. In the planning phase, you are developing many aspects such as the DDP which includes the GANTT chart. I think you should wait until everything including the DDP and the GANTT chart is done (or as much as possible) before doing source selection because you learn what you really need from that source. If you don’t know the time frame you need something, how can you ensure the source can provide? You have to know what you want to be able to say that that source works and can deliver or not. I think waiting until the executing process when you are actually using them ensures less things can change and affect your project.