dogman dan said:
If you do have some empty slots in your panel, then it will not be hard to wire up a 20 amps, 220 v plug nearby. You switch off the power, so it's safe to touch that stuff doing the work.
This assumes your breaker panel is in a garage. If its in a bedroom closet or something dumb, then it won't be convenient to tap in there.
Picture of your box, with the door open is a great starting point.
if you take the screws off that hold the cover onto the service entrance box then you will see there are two large wires coming in at the top and each of the breakers, one after the next, is connected to an alternating set of lugs sticking out from the back side of the box. each of those alternating lugs is one phase of the 240V. so there is 120V between each of those lugs and the ground wire, the bare copper wires, and the white ones on the 120v circuits are also at ground but they are called the 'common' wire.
you really do not need a separate set of breakers since you can connect the new outlet to two adjacent circuit breakers currently being used by putting the new wire under the screw along with the old wire to the old circuit.
to get the wire out of the service box, you have to punch out one of those multiringed holes around the perimeter you see where the wires exit the box. you need to use a grommet inside the metal hole and around the outside of the new wire. you can buy the grommet at the store. little plastic things with dog leg like hooks to hold it in the hole, then the wire goes through that.
find a place on the wall where you know from examination that you can run the wire out of the box to that spot on the wall and set a repair box there. you will have to cut the hole in the wall to fit the repair box but do not put the box in yet.
run your wire out of the service panel through the new hole and grommet over to where the repair box is since you will need to get your hands inside the wall to get the wire over there to the repair box. then punch out the opening in the repair box and pull your wire into the box, then put the repair box in the wall and tighten up the dog ears that hold it in place.
wire up your duplex outlet with the two wires, black and white on each side and put the bare copper ground on the ground leg. then go put the black wire on one of the breakers and put the white wire on the adjacent breaker so there is 240V across them and put the bare copper ground wire on the ground buss, what is called the service ground bar in the box.
then turn on the breakers again and you will then have a 240V duplex outlet. label it with red fingernail polish so you know which one it is later and so other idiots don't use it for 120V service. you can run an extension cord to your spot welder from there.