Hi,
I would look at the threads on tubs as well as our stickies on setting up glass tanks before buying anything really.
The main thing that jumps out at me is you really don't want to get a vivarium or enclosure that is too large as it can make them feel insecure. I know it is tempting to try and buy the enclosure an adult will need so you do not need to buy intermediate sizes but in reality it might just cause you more trouble than it is worth.
What kind of enclosures have you been looking at? And if you have any links to the shops etc you saw them in then I'm sure people will be glad to have a look at them and give any help they can.
Do you have a snake in mind? If so what size is it etc and do you know if it is wild caught, captive farmed or captive born and bred?
dr del