That is just a local motoring spares shop. They suppy parts, mostly to the trade, but will have a front counter where you can walk in and order parts etc. Most parts they sell are copy parts rather than Rover branded parts.
have you tried adjusting it before you go buying a new one?
Should be a 15mm bolt on the alty that you can adjust losen off and the move the alty to tighten the belt up.