Gates lists belts in half inch increments but I've found that I can order the next size up and it might be exactly a half inch larger or somewhat smaller or larger than the stated size. It seems that when you are making belts close is good enough. All you can do is to use the link provided to find the next size down and have your auto parts store order the belt and give it a try. The V-belts I normally use are the XL 25 series with 7xxx belts being sort of .41" wide and the 9xxx series being sort of .48" wide. My AC 1967 GTO used an XL25 9520 power steering belt and an XL 25 7490 alternator belt. My power steering belt is on page 19 of the web listing:
Gates belt sizes