Contact Webley, they sell them for about Ł10...
But beware.....
You'll need to put some sort of lubricant on so that it doesn't "lock" itself against the silencer, then when you twist the silencer off, the whole barrel shroud comes off in your hand...
Speaking from experience and people can back me up on this, if you don't need to do it, DONT DO IT, you will regret ever getting it done... I did....
Ian