You can certainly find an adult and if you search your local Craigslist you may even be able to find one free or dirt cheap with its cage included. Kids loose interest or leave for college and since a BP can live 30+ years the snake is often left with the parents who decide to rehome it.
As for the humidity and a screen top. Most of us cover a large portion with something to trap both heat and humidity. I used cardboard and created a cutout with windows for allow the heat and light from my CHE and heat lamp to pass through. I then covered the cardboard with foil. Others have used glass, plexi, foil tape, etc.
The biggest help to using a glass tank to give the snake the humidity it needs is to place moss inside the hides. I keep the moss damp during a shed and allow it to dry out during non shed times. This has given me perfect sheds in my glass tank.
Yes frozen rats or mice can be fed throughout its life.
A six month old BP is what I would consider pretty small. A 6ft female would be really big. Most BPs are 4-5 ft.