The Tensioner pulley for the alternator belt isn't sold seperatly and the whole part is quite expensive (£75
+).
However if you are ingenious and the tensioner isn't damaged then you can replace the bearing inside it.
You need to drill out the 3 "Rivets" holding the pulley halves together
Then replace the bearing (NTN part number 6203 LHA).
Then refix the two pulley halves together either by a new rivet or welding or bolting them together. The choice is yours.
I hope this is of use.
Tensioner pulley bearing replacement
Re: Tensioner pulley bearing replacement
Instead of bearing 6203 LHA you can substitute 6203 2RS instead which is more common and will do the same job.