Are you sure the solenoid is the problem?
Check the wiring. Make sure that the hot wire isn't broken or the connection to the wiring harness isn't broken. Is there current?
Remove the solenoid and see if the plunger is moving freely. Sometimes some liquid wrench can free it up.
It looks like the part has been discontinued. There may be a few places that have some stock.
Keep looking for the part.
When all else fails, cut the plunger off with a pair of side cutters.
Original Kohler Kit, Solenoid Valve R part 24-757-50-S from ProParts Direct. Kohler Engine parts shipped to your door. Call us with replacement part questions 800-305-9255.
If you need just the solenoid there is a lower cost option like the Honda part. This is the solenoid in the 24 757 50-S repair kit, just not the gaskets.