@miyako19 Oh I know. But in order to become a demon, Muzan must inject his cells to an individual. Tamayo was injected by him and after losing her link to him, she experimented on herself until she was able to turn Yushiro into a demon. In my understanding, Tamayo used her cells to turn Yushiro into a demon. Where else is she going to find Muzan cells when there are some within her already. She can go hunt down other demons but it can put her at risk of exposing herself to Muzan.
Tamayo cells is way different to Muzan cells but are fundamentally the same. Both cells can turn an individual into demons but due to the experimentations made by Tamayo, it can't be treated as the same Muzan cells.
It might be possible that Tamayo might have found the source of the Blue Spider Lily which was used to turn Muzan into a demon but it's very unlikely for that to happen due to the fact that Muzan can't even find it in his lifetime even with the use of demons to look for it. If he can't find it, there's no way in hell she can find it.