Where is High Sierra Installation file?

I am a professional musician. About 50% of my musical hardware still works perfectly with 32-bit driver. Many companies don't support still very good, expensive (flagship) models. New doesn't means better, faster. I need STABLE. This situation is sick! Now, I can't buy new Mac Pro 2019, because of 64-bit environment of Catalina or Big Sur (in 2021 Big High Sur etc.). I made a modernization of my old MacPro 2010 (flagship model). Now, after changing Graphic card with Metal function, 128 GB RAM, only SSD and M2 NVMe disc, I try install again new, pure OSX High Sierra or Mojave... but I can't find installation file. Any link on Apple support are inactive... Disaster.

If only a strategy of Apple is to change every year new OS with all equipment, so I ask: maybe will be better change OS every week? Why not? After all, our Earth has inexhaustible sources of energy and materials... I am VERY disappointed what are doing Apple. This is company, unfortunately, not for professionals par excellence. Yes, yes, my Dear Apple, take all our money! And what's next?

Apocalypsis... for Big Shure...