Getting stuff for baby

This is my third baby second girl we have most of the stuff from our daughter (our oldest). I need to get a double stroller due to my son only being a year old when she’s born but when should I start buying clothes for her we already have pretty much everything else and she will be taking my sons car seat because we will get him a bigger one but we don’t have newborn clothes aside from a couple outfits or blankets I’m 20 weeks now so just curious when everyone else started.