Many parts are available through some members here.
Solenoids and starters you want to ensure you use OEM parts as the aftermarkets just don't hold up. True with most wear rings as well.
In a hurry, you can install a lawn mower solenoid while waiting for a Seadoo replacement. Basically all the same as far as hot wire in, hot wire out, start wire and ground wire. Most solenoids NOT designed for marine are grounded when you bolt them on, if you use a lawn mower solenoid for example you will need to put the ground wire onto the mounting bracket for the ground.