GB, any oven that's going to put out a reasonable amount of heat will require a dedicated 220-240V circuit. I'm sure you're already aware of this, but, for typical 110V outlets (that you might plug a lamp into), those tend to be grouped together, in bunches, on the same circuit/wire going to the main panel. As you get into more powerful appliances, though, they need their own wiring to the panel, along with a special outlet. You can either temporarily borrow that dedicated circuit/outlet from another appliance (oven, dryer, larger A/C, etc.) with an appropriate plug adapter (if needed) and extension cord, or, you can add another circuit breaker to the panel.
Edit: After going into the details of adding a circuit breaker (see below), I did a little more research on extension cords and found that you can buy long high amp extension cords from welding supply houses. Adding a circuit breaker could still be a viable option, so I'll leave the instructions below, but I think a low gauge cord is the better choice.
The pizzamaster oven that I linked to earlier, for instance, at 3.63 kW at 230V, would pull almost 16 amps. That means that any outlet/extension cord combo rated to 20A would suffice. As you increase the length of the cable the gauge needs to increase, accordingly, so, if you could get away with a 50 ft. cord, that would give you a more portable cord than 75 ft. It should also save you some money.
When choosing extension cords, I think it's better to err a bit on the side of caution and go with a wire that's most likely a little too thick. You probably won't need 10 gauge at 50 ft., but that's probably what I'd get for a 16A device.
You can buy complete cords, but, it's cheaper to buy the cable and plugs separately, not to mention that you'll most likely need an assortment of plugs.
Here's the plugs you might need:http://en.wikipedia.org/wiki/File:NEMA_simplified_pins.svg
You probably don't need every flavor, and the 15A ones are ruled out, but I'd say you probably need around 8 plugs to cover your bases. At, I think, around $20 a plug, that's $160. If you have the plugs on hand, you can wire them as needed on site.
One other thing to keep in mind. If you do decide to go with the Pizzamaster (which, for countertops, seems to be noticeably superior to other brands), it's most likely going to be set up for European wiring. It's not the end of the world, but you'll probably have to take an extra step or too to get it working. If they are selling a lot of these ovens to the American market, I'm sure they'll have instructions.
Adding a circuit breaker may seem a bit scary, but, if you're careful and you make sure the main power switch is off, as well as avoiding the two hot wires going into the main, there's no danger involved. Turn the main power off, plug in the breaker, turn the main power back on. Since you can have the breaker wired before you arrive to the client, all you're basically doing when you get there is plugging the breaker in.
It's not rocket science. Here are two videos that show the process:
Both videos are loaded with disclaimers, as they should be, but the safety really boils down to two things- flick the main switch off and keep to the area where you're adding the breaker. The first video points out the wires that, after the main is turned off, still have juice/should be avoided.
They also spend some time on house wiring. You won't have to worry about that, since you're just running your wire to wherever your oven is. Other than purchasing the necessary circuit breaker, you're talking about 5 minutes worth of work on wiring and plugging in the breaker.
Now, this does require a little extra work on the front end. Since circuit breakers are brand specific, you've got to make a trip to the client before the event and see what brand/model of panel they have so you can get the right circuit breaker from Home Depot. You also need to confirm that their panel has a space for another breaker and that the main has enough amps to accommodate whatever amperage you're adding. It's just a matter of reading the number of amps printed on the lever of the main circuit breaker and subtracting all the amps from the numbers printed on all the other levers in the panel. If the resulting number is higher than the amperage you're adding, you're good to go.
When there's a will, there's a way. There is a huge safety factor involved, but, as long as you're conscientious, turn the power off, and avoid the two main wires, you'll be fine. You might find a really old house with a fuse box rather than circuit breakers, but they should still have a large appliance that you can utilize the outlet from.