While I see the area for confusion when considering the appropriate time to consider source selection for a particular project, I believe this process generally falls under the excitation phase. Having said this, I believe a good project manager should consider the source of materials earlier in the project’s life cycle such as in the planning phase. Similar to what Hiren was saying in his answer, the PM should have a general idea throughout the project yet, not waste too much time investing in potential vendors early on for a wide variety of reasons. As mentioned above, various changes could occur on the supplier’s side during the project life cycle so it is important to not invest too much time/trust into such a time variable asset. By taking preventative actions in anticipation in this way, change control, scope change control, and schedule control can exercised to make the process run as smoothly as possible. Ideally, this consideration of source selection throughout the project will matriculate into a confident decision when the project matures into the execution phase.