Some belt manufacturers list inner diameter some list outer diameter and some list both. Depending of how much you can lose from tensioning

For multi poly-v belts most part numbers will be..... number or ribs, (i'm guessing at 4 or 6 for alt belt??) and then the length (ie 1200mm) so as an example 4PK1200.

Can download many catalogues online for the likes of Gates, Dayco etc.... Gates sizes are often listed in inches so are basically K040472 which would be the equivelent part number for Gates.

I wouldn't try Halfords as they can be a bit dense, but most normal factors will have access to decent volumes af parts catalogues.