Also an option on Travis CI. Another option is getting Hercules, which can emulate a 390x, but, IIRC, falls short on some post z14 features more recent Linuxes rely upon. If you need time on an actual machine, there is the LinuxONE community cloud (Linux under z/VM) and IBM provides hardened (and expensive for 24x7 operation) Linux on Z instances (under KVM).

The IBM mainframe developer relations team is unusually friendly and approachable as well.