I don't think you can buy a "sump", though, hey, maybe some genius will decide to actually patent a sump someday for us fish-keepers. I'm sure we'd be glad to put out a few hundred bucks for such a setup!
The term is used in the hobby, but it is up to the hobbyst to make the sump, putting into it whatever they want to put into it such as: biological, mechanical filtration, skimmer, heater, etc..... Its a great opportunity for hobbyists to experiment without getting into too much trouble. So, if you want a "sump", then start out with a plain old glass aquarium [the bigger the better, but any size will do], run "in-out" tubing to and from tank and sump [using a pump to get water circulating from tank to sump and back again], then have fun deciding whatever the heck you want to put in it. If you don't like a bunch of bulky, unsightly equiptment in the main setup, the sump is a great place to put all that stuff! You can do most of your filtration in the sump and leave your tank alone [or almost alone].
Have fun!.....[Hey, this kinda put me in the mood to get going on my own "sump" LOL]
[This message has been edited by beth (edited 06-01-2000).]