If urgency is the case I would swing by local thrift stores and garage/yard sales. If buying online is the vital thing I would look at local facebook groups (garage sales and your city/state) that should bring some sites you can post ISO "in search of" postings. If budget is also an issue, I would set up an account on Freecycle or Trash Nothing and do a similar ISO post, include details like size and items needed (t shirts, sweatshirts, sweat pants, shorts, socks, pajamas, sneakers, etc.). If doing these things online is too difficult, in US, most public libraries can help with using computers and navigating the internet. Congrats on your new addition!