Playseat, Butt Kicker (used with Oculus Rift) modification

[ThugUK]

I have a Play Seat and Butt Kicker (actually a Pearl Throne Thumper) I use on my PC racing games (Project cars and Assetto Corsa) with VR (Oculus Rift) and wanting some help/advice on upgrading it.

At the moment its sat on the bedroom/computer room floor upstairs and when the Butt Kicker kicks in it vibrates quite badly down stairs.

At the moment I have it sat on foam to soak the vibrations up, but this has now compressed.

I was thinking of mounting the full gaming chair on springs. This would have 2 advantages, firstly it would soak up the vibrations, but it would also create more movement when the Butt Kicker activates, giving a better feeling of movement.

My question is, how/where (UK) can I get hold of springs and how can I mount them?

I thought possibly car engine valve springs (and washers). I could drill a hole through the wood and place a bolt through it, but not fasten it (to allow free movement), then attach the springs underneath. I would probably need around 6 for the seat (3 either side) and 2 for the front (as there is not much weight there). The problem is I don't know what strength of springs to use as it would be trial and error.
I then need to get hold of these springs/washers as cheap as possible.