I wonder how hard it'd really be to create something like that. I imagine the best thing would be to build a metal or water-sealed wood frame to surround and support the structure and then you'd have to buy a thick bottom plate (which you could probably get cheaper by buying a less desireable type of glass or plexi, since it'll be covered in substrate anyways). Then, you'd have to set custom-cut pieces of plass or plexi with a good silicone aquarium-safe sealant.
Depending on how big you wanted it to be... I bet making it yourself would be cheaper than trying to have someone else do it. But I like DIY projects. I'd probably love to experiment with creating something like this myself if there was a market for it around here... : /
It'd be lovely to make something like that box-shaped, polished wood above-ground pond that one guy listed... only with viewing panes cut out on the sides, don't you think?
The biggest challenge would be the stand. Making something that large and that wide to be able to support the broad bottom of such a big tank!! I imagine you'd almost have to make one big, or several small, concrete pedistles for it to sit on....
Heh heh... sorry. I think too much. ^_^;;