As already said,
It's important to know what type of monitor.
If you're just powering an lcd, grab the 110v that's powers your arcade PSU and you can tap off to an outlet or cut the end off the power cord and splice it.
If you are using an old arcade monitor like a wg4600, 4900, sanyo 20ez or similar, you will need to use an isolation transformer.
There are some newer and later model arcade CRTs that don't require an ISO transformer but if in doubt, it's best to use one.