https://www.coursera.org/professional-certificates/ibm-z-mai...
Also the MOSHIX mainframe YouTube channel has a lot of info, and helped me setup HERCULES emulator for my own little mainframe experience.
There are some free resources available that will allow you to get training but I haven’t tried them myself. IBM Z Xplore is worth a look as an example: https://www.ibm.com/products/z/resources/zxplore
I hope you find a way in, more mainframe developers and sysadmins(often called systemsprogrammers in the mainframe niche) are always needed.
Edit*: Spelling and grammar
They have it hooked in to VS Code now. It’s weirdly modern. And you get to play on a real z machine.
Recommendable summer/holiday tinkering project. It’s amazing how much and yet how little has changed in computing and transaction processing.
You can also look at the IBM Redbooks site[2]. Search for terms like Z/OS, MVS, CICS, DB/2, etc. and you'll find a lot of IBM books, whitepapers (well, they call them redpapers, but whatever) and so on.
The suggestions above are great if you're interested in learning about the technologies used on z/OS.
A vote for zeeframe's recommendation of the IBM Z Xplore course. Great stuff.
If you're looking for more reading material, I'd suggest the O'Reilly book "Modern Mainframe Development" book that covers a range of topics. (https://learning.oreilly.com/library/view/modern-mainframe-d...)
If you're looking for an on-the-job training approach that would enable a career change into the mainframe, Broadcom offers something called the Vitality program (https://mainframe.broadcom.com/beyondcode/vitality_program) that runs every year in North America, Europe, and Australia. It's a program that would equip you with serious credentials that could help you land one of those System Programmer jobs or similar.