Theoben Rapid Forum

Any suggestions on where I can get a strong valve spring please.

Why ? All you're doing with a stronger spring is asking for a bent valve stem.

if the valve's leaking replace it, if you're trying to reduce the air flow/power, reduce the reg pressure (if you have one) shorten the hammer spring & make sure it's running the correct hammer pip.
An economic consistent rapid is all about balance.

I can give you a stronger valve spring, but it'll not end well  ::)