If I get a 240v street light, how do you wire it up (I know its two phases with no neutral)?

The best would be to look inside, whether the ballast has 120V input and use that. Using across-phase connections for lighting in homes is against the code, because most control equipment (switches, protections, sockets,...) are just not designed to handle that connection safely for nonprofessional users (in the streetlight business these are solved by the training and license for all people accessing the lamp).

If I get a 240v street light, how do you wire it up (I know its two phases with no neutral)?

You can connect the metal body to groun and one of the two fases to the local current (crazy idea)

I think you need a transformer to connect the other phase.

The easiest way: see if the ballast has 120 taps. Or, if you have an electric dryer or range it will have a 240v plug. You can get a matching plug with cord at the hardware store.

As you don't seem to have basic electric understanding, I won't go into the other options, but you can also contact an electrician to have them install a 240v outlet.

Me, personally, I'd just source a 120v ballast, that's the best way to go for general use.

BTW a 240v fixture will use only 3 wires, line a, line b and earth. Don't bother trying to find a termination for a neutral/white wire, that's not used (if you use an appliance cord)

You can connect the metal body to groun and one of the two fases to the local current (crazy idea)

That should do nothing, as the metal body should be fully insulated from the working wiring. If it is not, itis time scrapping the ballast, because it is faulty.

And for the 240V for lighting in north America: The NEC does not allow to connect any lighting in residential areas to 240V, so better do not mess with it. There are good reasons behind those rules, so it is never bad idea to stick on them.
Ideal case is to find a 120V tap in the ballast. Very likely it is there, its wire could just be shortened and capped when the thing was originally installed. If it is an electronic ballast, check, whether it already does not operate on 120V as well.
Otherwise the only good option is to use a transformer to step the voltage up. The switch is then obviously on the 120V side, so that transformer becomes part of the light fixture.

There is not much reason to restrict lighting from 240v in residential other than it being mostly unnecessary. It is not any more dangerous than 120v. In a properly bonded system neutral and ground have the same potential. Your home will have line A and line B available in various places, in essence you theoretically could have 240vac available in a room (besides obviously appliance outlet)

240v is classified the same as 120v in USA.