Sump Pump Source?

I've got an Aquascape Designs system installed (almost ready to add water).

It's not set up to accommodate a bottom drain [meaning, plumbing a bottom
drain into the pump isn't accommodated]. Based on advice here, I installed
a bottom drain anyway, and stubbed in behind the skimmer (4"). The plan is
to use a separate 'sump pump' like thing, and once a week or so, cycle it it
to suck the junk off the bottom.

Any ideas as to where I might find an appropriate pump (places to search on
line?) It won't run continuously, but when it does run, I want it to really
pull the gunk out. Home depot didn't have anything appropriate.

Thanks.