You can find plexiglass at most places like Home Depot or Lowe's in a variety of sizes and thickness. Some places will cut it for you, some won't so phone ahead and ask.

The moss you are likely looking for is sphagnum moss in it's dry form. You should be able to find good sized bags of it at most any gardening center or major craft stores (I got mine at Michaels). You can mist it to help with humidity issues or create a humid hide but you don't want it sopping, dripping wet. Btw you want dry sphagnum moss not the sphagnum peat moss stuff. Here's what it looks like...

http://www.mosserlee.com/img/whatis2.jpg

As far as taking your snake outdoors, it's your call. Just know that the risks with certain species of snake as far as escape are greater than others and consider things outside like mole holes they might get down, if the yard has been treated or sprayed for weeds, a wandering cat showing up, etc. Also any reaction the neighbours might have. I've taken snakes out for a bit of sunshine but I rarely let them out of my hands and I don't do that all that often anymore.