is a link to a commercially-available unit (it's the 4th or 5th item down the page), but you can buy the materials at Wal-Mart and do your own for a lot less. All you need is an inline fuse holder, a switch, and some wire.
Remove the old fuse and wire the switch and an inline fuse across the terminals where the original plug-in fuse used to be. Make sure the switch and wire are rated to carry the amperage of the original fuse (or higher). The new, inline fuse should be on the hot side of the fuse you are bypassing. Otherwise the hot side of circuit will not be protected when the switch is turned off.