I mean they are multiple sizes but there are certainly standards. When they were more common I use to have boxes of them and could always find barrels to fit anything I needed and swap over the power supply to match whatever power I needed. Once in awhile you would get lucky and have both a matching plug and supply to match your need, like my electric piano uses my old Sega Genesis wall wart and plug because they matched voltage and barrel plug size and a cat chewed the original piano one and cause it to short and burn out.