if your feeling up to it you could try putting one together yourself as it'll work out alot cheaper.
you can get the glass cut at a local glazers then just use some silicone to fix the panes together. Pretty sure BigC pinned a topic somewhere showing how to do this?
alternatively there are a few companies around that'll build you one:
AC Aquatics
ND Aquatics
Poseidon's Palace
to name just 3.
Rob