Hello PeteS,
In my cases the usual scenario is that none of the single-source parts makes it, no matter from whom. Whenever possible I try to use standard logic chips, FETs, BJT and passives to do just about anything. There are designs still in production where close to 100 parts could have been replaced with a few chips and only a dozen passives. But I didn't and the clients are happy about it. Typically I never hear any mention of logistics problems again after such re-designs.
It's worse. For example, when I tried to obtain a partial reel of the Infineon FET BSP297 they really blew it. Didn't call back, promised to send samples but failed to do so, etc. It's not that I wanted freebies. I told them I would pay whatever it takes to get them to the US. Well, they lost that design-in and I haven't considered their other offerings since.
The sad thing is that some of them are real technological leaders. Infineon, for example, offers a great selection of hotrod RF transistors, diodes and other useful parts. Truly mouth-watering stuff. But what good does it do if you can't buy them at Digikey and they don't understand how marketing to design engineers works?
Then there is the poor performance of many of their web sites but that's a whole 'nother matter.