Rock Auto is where I got mine. If you are sure that's the number, put it in the search box.
If not, type in "belts" and it will come to a drop down for widths. Select your width and it will then list them in lengths.
They have a great selection of belts at approx. less than 50% of the price anywhere else I've found. Plus, every belt I've gotten from them was made in US. I'd advise getting two so you have the correct spare on hand.
Rock Auto has been my go to place recently. They also have great prices on NGK spark plugs.
Gates has weird part numbers and I would double check that part number you have. I have a B52 on my bench and it's actually 51" long. It's what I use on mine.