1987 T25 2.1 petrol: alternator belt

[Poppy]

My alternator belt seems to be slipping as the alternator picks up the charging load shortly after start-up. The tell-tale squeak disappears after about a minute. It looks to me as if the belt is narrower than it should be for the pulleys. Where can I get a new belt that's going to be reliably the right one? (I've recently had a lot of work done including replacing the old PAS steel tubes with hoses, and I'm guessing that the mechanics "helpfully" changed the belt!)

[T3luestar]

http://www.brickwerks.co.uk" onclick="window.open(this.href);return false;