If you want to keep it standard, then the only manufacturer of the self levelling rear shocks is Vauxhall / GM - no other manufacturer made them for the aftermarket; presumably due to likely low demand.
Munro do a ride levellers kit, but they work off the same principle as the old CD Cavaliers in the late 80's - there was an air valve mounted on the boot / tailgate aperture which could raise or lower the desired ride height by using an airline or manual pump. Not ideal really. The height was adjusted manually and not automatically as the standard system dependent on weight in the car.
If you are not in a rush for them, they do crop up occasionally if you are prepared to search online. I have bought several for saloons and estates over recent years, 'just in case' and all have been under £50 each.