From other somatic cells, and ultimately from the tissue stem cell population. That population is impacted by aging, as you point out. However, the effects of replacing 1% of tissues over some period of time are going to be negligible in comparison to the effects of having 1% senescent cells, given that those senescent cells and the inflammatory, harmful signaling they generate are one of the reasons why the stem cell population is less active, and regeneration less effective.