at a number of places online
check eBay for the seller whitenile333 (in fact, he has 2x2x4 for $98 shipped online now). I got my first cage from him and still use it for a variety of things. Reptiledepot.com, www.screencages.com and of course Tyler at bluebeastreptile.com have screen cages
most of us use 2'x2'x4' all screen cages. Many have the flip panel at the bottom, but for Cyrus I switched it with the top panel so he has a little door to get in and out of his cage to his "gym"

If you get the 2x2x4 I suggest getting one with the panel at top and bottom (one flips out and the other is stationary) as it gives better support on such a tall cage.
hope this helps!
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Chameleon Help & Resource Info
1.0 Nosy Be Panther Chameleon - Cyrus
0.1 Veiled Chameleon - Luna. She's now hanging from her big jungle gym in the sky
1.0 Beardie - Darwin
1.1.1 Side-blotched lizards - Ana and Stan
0.2 felines - Kyndra and Lita
0.1 African Clawed Frog - Skippy
0.1 Chilean Rose Hair Tarantula - Rosa Leigh, Died 4/21/06 
0.1 Goliath Bird-Eater Tarantula - Natasha, donated to science 4/4/06
?.? Pinktoe Tarantula - no name yet