I probably have the same filter, but I have not needed one yet. But what I would do, depending on what sort of stores exist in your area, is go to a Cummins shop -- check the Yellow Pages. You could also telephone a Blue Bird dealer, who may be able to get you a part number. Last I looked, the Blue Bird factory web site had a list of dealers. Once you have a part number, any "serious" auto parts store should be able to order the filter. NAPA auto parts stores are big on commercial parts. "Floor mats and stereos" stores like Kragen, less so.
Also, take a good look at the old filter -- it may have a part number on it.
A good parts man may even be able to get you the right filter by measuring the old one.