Ask HN: How do you find and validate suppliers?

I need some custom steel fabrication done for a personal project. Local metal shops were way too expensive, so I tried Alibaba. I submitted an RFQ and got a couple dozen responses. Prices were anywhere from $200-$1000. They've all offered to send samples, but at a cost that makes it impractical to get more than one for my purposes.

This got me wondering: in the corporate world, how are suppliers found, chosen, vetted, etc? Does it make a difference if you're operating at startup scale (say Framework) vs FAANG scale? I'm not specifically wondering about metal work; curious about electronics too.