Windows or Doors For Treehouse Project (Shasta Lake)

Hi there, im building a treehouse for my 5 year old and looking for some building supplies.
Especially looking for some windows and or doors.
Ive got most of the wood i need but want to find some cheap or free windows and doors before i start framing it up so i can build them in.
Doesnt have to be anything special, just a fun project for my son.
Thanks for your considering Im located in shasta lake and can come pick them up.