First of all a correct size belt grips on the sides. Not touching the bottom at all. Belts are all standard widths on the top, or widest part of belt .All belts are available in many sizes.
However I am talking about industrial belts not automotive . An "a" belt is 1/2 inch wide on top. Available in almost any length. Graingers, Kaman bearing ,Motion industries etc. I work on electric motors for a living & although you might not find your belt in Auto zone or Pep boys it is available.