There are many websites available on this forum that sell cages and supplies.
If you go to the top of the forum page there is a link (first one basically) called "Breeders/Dealers/Stores"
That page has many websites for reptile supplies. Im sure you can find a cage there.
If you want to build your own cage I would check out this website.
http://www.chameleonparadise.com/2x2x4DIYCage.htm
He has detailed instructions to building a 2x4x4 cage. It says for outdoors but can be used inside as well.
LLL Reptiles has a good selection of cages as well. I am not sure on pricing but I found both my cages there and are wuite happy with them. I personally use Reptariums. They are PVC tube cages with a screen mesh cover. I have 2 and have been wuite happy with them.
http://lllreptile.com/v2/content/catalog/?section_string=cage-supplies
I would recommend at LEAST a 100 gallon reptarium or larger. The bigger the better since he will grow into it. I have my male in a 175 gallon and he loves it.
You can buy whole cage packages from them or just the cage by itself depending on what supplies you want.
Also be sure to check eBay, I have seen people selling reptariums on there lately for a decent price as well.
Look around the breeders/dealers/stores page though, im sure that you can find somewhere that has the perfect cage your looking for!
Good luck and hope this helped!
Erin