Hi there
there is a vivarium kit i know of called the Viquarium by Tetrafauna, it has a waterfall and filter built into the kit. i would say it would be good for a 20 gallon long. One way of building a vivarium (and saving some money) is collecting large rocks and put them in the middle of the aquarium so it makes a "wall" put aquarium gravel on the "land" side, put mesh down and substrate and presto, you have a vivarium!

Good luck on your viv search!