I see the generic link, but here’s my take on switcher sourcing.
First, check distribution for ‘in stock’ parts. The ones that are there are popular and will tend to be re-ordered, helping ensure a steady supply.
Second, choose older regulators that have as many second sources as possible, and try to make your design tolerant of differences.
This means perhaps eschewing specialty multi-output types or latest-and-greatest technology in favor of older, single-output parts.
For example, for 1-2A stuff I long ago settled on a common SOT23-6 synchronous type that has footprint compatible devices from US, Taiwan and China sources. Sometimes these have small pinout variations, so strap options can help.
Bigger regulators? Might need to consider external switch types using mature controller chips and FETs. Or perhaps using standardized POL modules.
Third, everyone has their favorite vendors. I personally try sourcing MPS, Diodes Inc, and TI first, followed by Maxim and Analog Devices. These companies are in it to win it.
I used to use Intersil, but now that they’re part of Renesas, be careful: Renesas tends to aggressively purge SKUs from its product lines in my experience.
On the other hand, International Rectifier is part of Siemens now. They’re a good choice for automotive/high rel and have a commitment to long term sourcing.
At the risk of self-contradiction, emerging vendors looking for growth, like AMS for example, might be easier to work with.
Finally, look for BOM leverage. If there are several different parts that you can source from the same vendor sometimes they will give you priority for your low-value devices. TI was big on this in my experience. Never hurts to ask.