Hello Mathew,
My suggestions will relate parts rather than procedures:-
If you aren’t buying genuine VAG parts at discount, then I would suggest you buy Gates kits which will include belt, tensioner and water pump (if belt driven).
As Kev says, EBay would be one source, but if I was in your situation, I would want to ensure that what you buy off EBay is not ‘old stock’ , since belts should be renewed every five years, and you could find belts may already be a couple or 3 years old before they are even fitted!
I have used Car Parts in Motion to good effect and at reasonable prices, so it might be worth checking there. I would also renew the serpentine belt and tensioner at the same time.
Hope some of this helps.
Kind regards,
Gareth.