I have a Toyota Alphard 2.4 Hybrid 4WD and want to put heavier/stronger shock absorbers on it, any ideas

You can get stronger (firmer) springs, I think someone was selling a pair on here recently for a good price. You’ll need to speak to a suspension expert about modding the damper units; I’d guess that if you’re not changing the firmness too much then you can probably stick with the existing dampers, but if you’re going to make a big change to spring firmness then you might need to change the dampers too. Suspension is a complex field, best speak to a specialist company as even small changes can have a big impact on handling and ride.