You’ll probably find smaller windows easier to work with than large ones.  Junkyards and reclamation yards are a good place to start looking. Ebay usually has some old lots of windows for sale by collection. Or you could just start checking dumpsters in your neighborhood. If you are using old windows, you need to be aware of some hazards such as splintered wood and nails sticking out.  Check your window carefully before handling it and be sure to use tough gloves.    {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/c\/c9\/Make-Garden-Art-with-Old-Windows-Step-2Bullet1.jpg\/v4-460px-Make-Garden-Art-with-Old-Windows-Step-2Bullet1.jpg","bigUrl":"\/images\/thumb\/c\/c9\/Make-Garden-Art-with-Old-Windows-Step-2Bullet1.jpg\/aid5045819-v4-728px-Make-Garden-Art-with-Old-Windows-Step-2Bullet1.jpg","smallWidth":460,"smallHeight":345,"bigWidth":"728","bigHeight":"546","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}  Be very cautious handling glass, especially if any panes are broken. Some panes may be loose. It will be safer if you can make these secure by using a window sealant before you begin working with your window creatively. You need to be cautious of woodworms and termites if you are bringing second hand furniture into your home.  Look out for little holes and piles of frass (termite excrement). Fumigation works best to get rid of them, or use a DIY chemical treatment, and seal up any holes.    {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/e\/ec\/Make-Garden-Art-with-Old-Windows-Step-3Bullet2.jpg\/v4-460px-Make-Garden-Art-with-Old-Windows-Step-3Bullet2.jpg","bigUrl":"\/images\/thumb\/e\/ec\/Make-Garden-Art-with-Old-Windows-Step-3Bullet2.jpg\/aid5045819-v4-728px-Make-Garden-Art-with-Old-Windows-Step-3Bullet2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":"728","bigHeight":"546","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}  If you leave the bugs in the wood, they will eat your art and possibly spread throughout your house.    {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/b\/b3\/Make-Garden-Art-with-Old-Windows-Step-3Bullet3.jpg\/v4-460px-Make-Garden-Art-with-Old-Windows-Step-3Bullet3.jpg","bigUrl":"\/images\/thumb\/b\/b3\/Make-Garden-Art-with-Old-Windows-Step-3Bullet3.jpg\/aid5045819-v4-728px-Make-Garden-Art-with-Old-Windows-Step-3Bullet3.jpg","smallWidth":460,"smallHeight":345,"bigWidth":"728","bigHeight":"546","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}

Summary:
Choose small windows to make creating your art easier. Make your windows safe before painting. Beware of bugs to make sure your art is not eaten.