Have you ever thought of making one yourself? I did and apart from costing very little It was a lot of fun.
It started off as two redundant bedside cabinets, I screwed them both together, I used the drawer fronts to make a door for his 'dark' side and put a couple of pieces of glass as you can see.
http://farm3.static.flickr.com/2287/2521974453_c763704963_o.jpg
http://farm3.static.flickr.com/2178/2522789606_e9756b2942_o.jpg
This made an app. 30 gallon vivarium.
When he grew to about size yours is I just added an extension on the top of the right hand cabinet, this is the only thing I bought, a piece of covered chipboard from B&Q.
http://farm4.static.flickr.com/3653/3485630786_f3fbed1fc3_b.jpg
Now it's 45 - 50 gallon
I've still got a bit of tidying up to do on it but I'm pleased with it. If you want to make a 'posher' one you might consider using/buying an old glass cabinet.