The approaches taken by Randy and Dave are probably the simplest solutions. If you want both simple and cheap, you could make up your own using an extension cord, switch and plastic box from Home Despot or an electrical supplier. And I like the idea of using one of those large round magnets from Harbor Freight so you have some flexibility on where it is mounted.
But let me raise one issue - does your lathe have a built-in worklight. If you go the simple route, the switch will control both the lathe and the worklight at the same time. I have a worklight on my lathe, and I prefer to leave it on the entire time I'm working at the lathe and not just while I am actively turning. My approach was to mount an outlet strip on the wall with receptacles for the lathe, for the drill that I use (only occasionally) for power sanding, and for an overhead fan that blows dust down and to the back of the lathe (and away from me). I can use the switch on the outlet strip to turn off everything when I'm not in the shop. When I turn on the outlet switch, that powers the lathe and turns on the worklight. The lathe itself is controlled from its built-in switch.
So if your lathe is like mine and you want your auxiliary switch to control only the lathe motor and not the worklight, you would need to cobble something together that is integrated into the internal circuitry of the lathe. That's not difficult.
I added a forward/reverse switch on my VS midi-lathe that is simply a double-pole, double-though toggle switch in a plastic box that attaches to the back of the lathe tailstock with a large magnet from Harbor Freight. My lathe has a cable from the DC motor that plugs into the VS box using a standard C13 connector. So I bought an extension cord with a C13 connector on one end and a C14 connector on the other end, cut it in half, and wired the switch in the middle so that it reverses the polarity of the DC voltage supplied to the motor, thereby reversing the direction of rotation for sanding. The switch has a middle-off position but I would never use it as an emergency off switch because it's too easy to pass through that middle off position to change rotation direction (which is also why I mount it on the back of the lathe where I have to think about what I am doing when I use it). But the same concept could be used with a different switch (a double-pole, single-throw) to create a movable remote on-off switch. And if you want to get fancy, you could even put two switches in series so that you have both a reversing switch and a remote control switch. If you were to do that, I would suggest using separate boxes for the two switches, and enough cable so that they can be separated enough that you can't inadvertently grab the reversing switch when you need to stop the lathe in an emergency.
