It depends on whether AEP deal direct or through a wholesaler or import agent.AEP told me they get their spheres from IFHS in Belgium, and I bought comfort spheres from them last year which I'm happy with (although not listed by AEP). If you look on the IFHS website and PDF catalogue they appear to still list a choice for the BX, so I wonder whether "our supplier has superseded all the spheres to the one for the BX" is more a case that AEP have rationalised their range to avoid keeping slow moving parts in stock...
My brother has found that some obsolete bicycle tyre sizes for the UK are not listed here but are available abroad where the sizes are still in demand. Again, once he knew what questions to ask he was able to obtain these sizes for certain customers with large numbers of old bicycles