Run into this problem myself quite a bit. We have an AS400 server at a site I manage the IT for, it runs an old ERP system they still use but we have medium term plans to replace it depending on how rollout goes at other sites. We have hardware support for the servers luckily but don't have support for the software any longer.
Only problems i've had in the past couple of years have been overheating and network connection dropping out. The printers cause me no end of problems though and the resellers charge a fortune for replacements (which still cost less than it would for a call out and repair).
Have never been able to find anything coherent online like tutorials or the like. Even though they are dead end systems there is still quite a market for support, maintenance etc, and even though our support contract isnt with IBM it is their engineers that come out for problems so its probably in their best interests to keep information scarce.