O.T. Bad power supply?

I have a Dell XPS 8500, with Windows 7 Professional, SP1,
with Spywareblaster, Malwarebytes, Avast , Windows Defender
and Windows firewall.

(1) TB HD
Intel (R) Core (TM) i7-33-3770 CPU @ 3.40 GHz
Ram 12.0 GB
System type : 64-bit operating system

I also have

I have a Dell Optiplex 780 Tower, with Windows 7 Professional,
SP1, with Spywareblaster, Malwarebytes, Avast , Windows Defender
and Windows firewall.

Intel (R) Core 2 Duo 2.93 GHz
4GB RAM, 750 GB HD
System type : 64-bit operating system

and (external hard drives)

(8500)
WD BLACK SERIES WD2003FZEX 2TB 7200
RPM 64MB Cache SATA 6.0Gb/s 3.5" Internal
Hard Drive

(780)
Seagate Desktop HDD ST2000DM001 2TB 64MB
Cache SATA 6.0Gb/s 3.5"
Internal Hard Drive

Today when powering up the computers the 780 kept trying to
initiate the monitor but it kept starting/stopping and
wouldn't power on.

So it looks as if I may have to buy a new power supply or
could it be something else?

Are there any thoughts/recommendations where to buy a OEM
power supply?

Thoughts/Suggestions

Thanks,
Robert