Compatible PSU for Dell T3500

I have a dodgy T3500 PSU that I want to replace with a new and maybe larger PSU maybe to 825W or 1100w

These power supplies are rare on ebay, so I'm looking for an alternative and suggestions?

The power supply has been running ok for months, but every now and then it does power on. I've tried

1) PSU test button - goes on for a while and the off when i let go

2) Disconnected everything - trying to power on with each disconnection

3) Air dusted the whole unit

4) Reconnected everything and tried to power on at each re -connection.

It will come good at some point, but the sequence of event to make that happen I haven't been able to determine